Spraying and Spray Guns

[Bob Connor]

I've been chatting with Allen Macfarlen about buying a new spray gun and the tips that he gave me were brilliant so I thought I'd copy them here as a resource for everyone.

I found a Sata Minijet 4 for a reasonable price and these were Allen's comments.

Allen -

The price of the gun is really good. I would grab it no matter what.

I don't know if its going to spray the shellac well or not for you. There are heaps of variables in gun set up and spraying style as well as air pressure.

The gun is designed to spray automotive paint, and it would be thinned to the same consistency as what you are using. Perhaps even a bit thicker, and it does that brilliantly.

All a spray gun is designed to do is atomize paint into small particles that aren't too large (heavy looking surface) and not to small either (dries before it reaches the surface). You can vary the way a gun does this by several ways.

1. Air pressure, lower has less atomization. Do this if product is too thin. Higher if too thick.Small adjustments can make big differences.

2. Fluid tip and air cap. Larger for thicker material, smaller for thinner.

3. Fluid needle adjustment. In for thick material (less has to be atomized) out for thin (more can be atomized).

4. Thin the paint more or less to bring it into line with your equipment and spraying style.

5. Spraying distance and speed.This is probably the biggest one. No matter how well a gun is designed and set up this will make or break the finish that you get. Takes a bit of practice, but you adjust this by looking at the wet edge you are spraying and observe how the finish is laying down. I would get my apprentices to fill their gun with water and practice laying a nice even wet coat on an old bonnet. They think that it is too easy until they try it.

The tip on the gun was designated as an SR (spot repair) and I didn't know whether it would be suitable for instrument finishing.

Allen -

The gun is designed as a spot repair gun for the automotive trade. It's the small size. No good at all for spraying a complete Landcruiser.

We would use a gun this size for doing door jams, fender skirts etc. Anywhere that we would only want to make up a small amount of paint and have the convenience of a small gun.

It should be good for guitars. You want the pot to be able to hold enough product to get one coat on the intended surfaces. It's not necessary to have enough for 3 coats, 'cause your going to let the surface flash off an appropriate amount of time before you spray some more.

Gives you time to refill and let you have a little break.

The tip should be fine to use on a guitar. You aren't trying to paint anything huge here. The rest would be controlled by air pressure, solvent, spray technique etc. The fluid nozzle, tip and air cap will be sold as a unit, it you were wanting to change them, and they will be pretty expensive. Essentially they are the spray gun, the rest is there just to make those pieces work.

I'd be glad to give anyone pointers on spraying. It's a lot easier when I've got them right beside me, but I should be able to clear a few things up. Most people haven't got a clue as to what all the knobs are for on the gun, and just knowing what they do makes a huge difference in your ability to trouble shoot spray problems.

So I guess I need to know what those knobs actually do Allen.

I've just searched every thread on the OLF for this type of information but Allen has nailed it all in a couple of PM's.

So, I owe him lots of beer.

I'm sure a few other people will have some questions too.

Thanks Allen

Bob

[Allen]

Here's a picture of a Sata Mini. All spray guns will have some arrangement of either 2 or 3 knobs depending on it's design and quality/price range.



The AIR PRESSURE KNOB is usually only found on high end spray guns, but should not be used as the only means of knocking down the air pressure from your compressor, especially if you have say 90 + psi coming out of it. This valve is mostly used for fine adjustments by the operator when they are spaying.

The SPREADER VALVE is either located as in this picture, or is above the FLUID NEEDLE (usually in siphon feed guns). You use this to change the width of the spray pattern. All the way in and you will get a small round pattern, all the way out and you will get a wide, flattish pattern.

The FLUID NEEDLE adjusts how much material is released into the air stream when you pull the trigger.

Initial set up should start as follows.

1. With gun empty, turn the fluid needle almost all the way out. Pull the trigger and then start to screw the fluid needle in until you feel it come up against the trigger and start to push against the trigger. I usually then scribe a line on the knob so that I know where fully open is, because small adjustments from here make big differences.

2. Turn fill pot with sprayable product. Water, or thinners is good to start with.

3. Set air pressure to around 45 psi at the gun. Air lines will have pressure drop so it's a good idea to have a gauge at the gun if your not sure.

4. Take a test spray against some cardboard or similar. Adjust SPREADER KNOB to see its effect.

Will continue later. Gotta get off to work.

[graham mcdonald]

Bob,

The trick with spraying shellac/french polish is absolutely minimal air pressure set on the gun. If you spray at pressure that you would use for nitro the shellac will dry before it hits the surface and turn into unsightly, uneven little brown lumps. I keep one gun for shellac, used mainly as a sealer/colour coat on maple and the air valve knob is opened just enough to get the liquid out of the pot.

Both my guns are just $20 cheapies from one of the automotive supply shops and they work fine, powered by a $100 compressor. A regulator/oil trap is very useful as these direct drive compressors do tend to leak some of the lubricating oil into the air flow. This setup would be not be sufficient if I was in a production situation but for spraying a couple of instruments every couple of months or so it is very practical. Just wait until there is a bit of a breeze so as not to annoy the neighbours

cheers

graham

[Kim]

Thanks guys, this is a VERY usefull thread indeed. Once Allen has made the rest of his input, I will be saving this one to disk for sure.

Cheers

Kim

[Allen]

Ok, here is some more info for you. A bit of the Readers Digest Condensed Version.

It's useful when learning to use cardboard or similar to practice spraying on. I recommend you use water to start with, but if your up on spraying you can use your finish of choice.

Explanations follow each image.



With the SPREADER VALVE all the way in, air does not come out of the little holes at the out side edges of the horns on the air cap. You should get a spray pattern something like the top dot. Depends on air pressure and paint viscosity, but you get the idea. There is NO air breaking up the paint stream into a wider fan like pattern.

With the SPREADER VALVE all the way out, the maximum amount of air that the gun is designed for is directed to the holes at the end of the horns of the air cap. The air coming out of the air horns will break up the paint stream and make a fan like pattern. You should get something like the bottom image but a little thinner at top and bottom. This was the best I could do in Photoshop.



Now here we have a problem. The pattern on the left shows an uneven spray pattern that is going to make sure that even the best painter will get runs. You need to remedy this or you'll never get a good finish. It's caused by either a dirty fluid tip/air cap or the fluid tip/air cap is damaged.

The image on the right is caused by too high air pressure for the product being sprayed or some times not enough product. Check that your FLUID ADJUSTMENT is all the way out. It will leave dry stripes in the finish as well as helping to make runs. Drop your air pressure and see if this helps. You may also need to turn in the SPREADER VALVE slightly.



If you have a spray pattern that is now correct you can try spraying a test panel. Here I've used different colors to show how much over lap that you use to get even coverage. The red lines show where the center of the spray pattern is on each pass. If you look closely, with each pass of the gun, the center of the spray pattern is over the edge of the previous one.

Everyones spraying style and speed is different and various finishes require slightly different techniques. As a starting point I would suggest the the spay gun be held approximately 8 inches from the surface to be sprayed. If you spread your hand out wide, the distance from the tip of your thumb to the tip of your pinky is close enough.

Now, start the movement of the gun out side of where you are going to spray, as the gun comes over the panel pull the trigger. This means that the gun is already moving when paint starts to hit the surface. Continue passing the gun over the surface and just as it leaves the surface let the trigger go. Be sure to keep the gun square to the surface. Don't let the gun follow an arch as you spray. That is, don't let it start one distance, then get closer in the middle of your pass, and then get further away at the end. We only use this technique when blending in finishes, but this is quite and advanced and takes a fair bit of skill. I can try to explain is someone needs to know how to do this.

I can't emphasize this enough. The gun must be moving when you pull the trigger or you will create a thick spot of paint and usually cause a run, and it must stay a constant distance from the surface.

Each subsequent pass of the spray gun has a 50% overlap of the preceding one and sprayed in a right-left then left right direction. That is reverse the direction you are spraying with each pass of the gun. GETTING THIS RIGHT IS MOST IMPORTANT. Practice with water on cardboard until this becomes second nature.

While trying to keep all this straight, you need to watch the surface as you are spraying. I try to get the light just right so that I can see the gloss of the wet surface as I'm spraying. I watch the wet edge and adjust my speed and distance according to what I see. You want to get a surface that is just wetting out so that its glossy. More than that is no good at all. You are likely going to get a heavy look that is full of orange peel or runs.

I will also add that there is going to be huge variations in air pressure, depending on what you are spraying as Graham mentioned. Thin finishes will be able to be sprayed with lower air pressure. Perhaps as low as 25 psi. Heavy ones will require as much as 60 psi. This also depends on your spray gun. I spray Miratone thinned 50/50 with slow thinners at 40-45 psi out of my gravity feed. If I was to use a siphon feed I would have to raise that by about 5 psi.

If your technique is not up to snuff, or even if it is, it's always worth checking out your guns spray pattern, and doing some practice spray panels. I sprayed cars full time for over 25 years, and I still do this, mostly as a precaution whenever I go back into the spray booth.

There is heaps more to getting a good finish off the gun. I anyone has specific questions please feel free. I could write a text book, but there is nothing like practice.

[Dave White]

Allen,

Great stuff - thanks!!

Bob,

This seems an ideal candidate for the Tutorials section.

[Bob Connor]

Excellent Allen.

Could you give us a run down on cleaning and storage of the gun please.


Dave - I think we'll let it runs it's course here then move it into the tutorials section but I agree this is a fantastic resource and one of the best spraying tutorials I've seen.

Bob

[kiwigeo]

Allen, Bob,

Where abouts do you source your sata spray guns?

Cheers Martin

[Allen]

If your looking for new, then any automotive paint supplier will be able to supply high quality spray guns

I like Sata, but there are heaps of other great brands as well. Devilibis is another one that many spray painters like. Be warned though that they are pricey, and designed for day to day use in a production environment. You can get good results from far less expensive guns by just knowing how to set them up and good technique.

I would be a little apprehensive about spending big money on one from eBay or such. If it's used it could be abused,and I would want to see the spray pattern. If the gun has ever taken a blow to the air cap or fluid tip (very easy to do with careless handling) then you will get a spray pattern like the left one in the second image, that can't be corrected without purchasing a new fluid tip/air cap and needle combination. These will cost well over $100 and depending on the gun can be much more. The last time I had to replace mine in an Optima Gun it was $470 Cdn.

To clean a gun, I put a splash of thinner in the empty pot and give it a good shake, (with the lid on) . Then spray out some of the thinner and take a rag and smother the air cap for a few seconds, remove the rag and let some more thinner spray out, then smother with the rag. What this does is force air back through the gun, and back flushes the fluid passages. Put fresh thinners in the pot and repeat 2 or 3 times.

Never submerge a spray gun in solvent. You do not want solvent, and especially contaminated solvent to get into the air passages of the gun. The only place that needs to get flushed out and cleaned are the fluid passages.Thats what you are doing by flushing out the gun several times with a splash of clean solvent each time. In a gravity feed, that is from the pot down to the fluid tip. In a siphon feed, thats up the pick up tube and out the fluid tip.

You can use a rag with solvent, brushes etc. to clean the body and pot of the spray gun.

If I plan on using the gun in the near future I will take the air cap off and put it into the pot with enough fresh solvent to cover the air cap. Put on the lid and store the gun carefully, (I have a stand that holds the gun upright) and make sure that you don't bang the fluid tip. They're very delicate.

If your not going to use the gun for some time, then take the air cap off, then pull out the fluid needle (back off the fluid valve knob until it comes out. There will be a needle with a spring. Then take out the fluid tip. You will need the appropriate size wrench to do this. Give them all a clean in fresh solvent, then assemble the gun. DO NOT USE ANY LUBRICATION ON ANY PART OF THE GUN. Old spray guns use to use a leather gasket for some of the seals, that might have needed a drop of mineral oil, but no modern spray guns use leather gasket.

Pull the trigger all the way in and then adjust the fluid valve until it just wants to start to push against the trigger. The gun can now be stored indefinitely and is ready to go the next time around.

[Allen]

I haven't yet explained why there is a FLUID ADJUSTMENT VALVE yet so here goes. Also some tips on AIR PRESSURE.

When you purchase a spray gun, there is usually several options of fluid tip/air cap combinations. Each automotive paint manufacturer has tested their product with all gun combinations that they think will be used in the real world. They then will have a recommendation as to the best fluid tip size for their particular product in a particular spray gun. This will give optimum results at 20 degrees C and 45% RH. This is the agreed standard. Seems familiar doesn't it.

So a spray gun has an optimum set up, but we still can spray various materials through it with different viscosities, just by adjusting the FLUID VALVE and AIR PRESSURE.

Painting in the real world is never that simple though. If it's colder than 20 the paint will be thicker, and if warmer it will be thinner.

We still only want to apply the correct amount of finish with each coat, so if it's colder than 20, then we will turn IN the FLUID VALVE so it reduces the amount that you can pull the trigger and reduces the amount of paint that is able to get past the needle to the fluid tip. That's why I said to mark the knob of the FLUID VALVE so you will have an idea how much of an adjustment you have made.

I find that if it's been cold out (remember I use to be a Canuck) then the paint will be quite thick and I would turn in the FLUID VALVE 2 full turns. You may also need to bump up your air pressure just a bit. I would estimate 2-3 pounds. Never more than 5.

You might be saying to yourself "If the paint is thick, then it's going to come out the tip slower, so why have less"?

The reason is that thick, cold paint also doesn't flow well, at least initially. It will once it warms up though.

You may have experienced this. You spray a panel with paint that isn't warm enough onto a panel that just might be a bit warmer than the paint. You will end up putting on more paint than you should because it isn't going to flow like it would if it was 20 degrees.

You think that you have a nice even coat that's going to look great. However you come back in a few minutes to see some great sags or runs in the finish. This happens because the paint went on too heavy and sort of seizes on the surface. Because the paint is colder than it should be, the solvents don't evaporate as fast as they should. Then the paint starts to warm up and gets thinner and wants to flow. How many times has this happened to you?

Now if it's quite hot out, then you are going to be struggling to keep a wet edge. If spraying a finish where you have a choice of solvent, then opt for a slow one. You might even need Retarder, which is just ultra slow solvent.

You might need to turn IN the FLUID VALVE a bit, but I find that I need to adjust the air pressure more often. Because it's hot and the paint is thinner than optimum, you will need to DROP the AIR PRESSURE a bit. No more than 5 psi. from normal. With it being hot, solvents will want to evaporate quicker, so you have to move quick to keep a wet edge.

This takes me back to the test patterns. One of the things that you are looking for in the test pattern that you spray is the atomization that you are getting.You want a nice even spray pattern that looks wet. There should be a nice even fog of fine paint droplets around your spray out. If you see uneven atomization, ie. large droplets then bump UP the AIR PRESSURE a bit and try again. You could also try turning IN the FLUID VALVE.

If you see a dry, powdery spray pattern, then your AIR PRESSURE is to high. Drop it and try again. With shellac, you only have the choice of one solvent, so you will be quite dependent on AIR PRESSURE, and technique. If you still have problems, then I suggest that you add more solvent. It doesn't change the shellac, just means that you will need to apply more coats.

One final note. Use a air pressure gauge at the gun. Pressure drop in air lines can be huge. While it might say 75 psi at the tank, with a 1/4' inside diameter air line that is 30 feet long, you will be lucky to have 35 psi at the gun.

[Kim]

BUMP because it's brilliant.

Thanks Allen

[Taffy Evans]

Hi Allen
Thanks for the helpful info you have posted on spraying. Since day one it's been an issue with me, not getting a consistant outcome off the gun. I have had many good finishes but too often in the past it involved many hours of touch up and buffing. I have bought and still have 9 guns of diferent types. Brands include Devilblis, Binks, Scorpion, Iwata and some no namers. Not all were a waste as some are for colour and some for clear finishes. I also have 5 air brushes.

I think my problem in the past has been that, [besides not l taking the time to practice as you say] is that over the years I have [due to the pressures of repairs] sprayed only a couple of times a year so if I got it right in February I'd forget what I did by the next guitar in October.

But now that I am putting more time into building and getting better results with my finishing, your info will help me to finally put pleasure into that side of the building process. Thanks Allen.

[Allen]

I should run a course hey.

I'm glad that your guys are finding this stuff helpful. It's really the only part of building that I don't have to think about, and struggle with. Maybe one day the rest of the building process will become second nature.

[Rod True]

Allen, thank you so much for all this amazing detail. This is one of if not the best detail on spraying a lacquer finish on the net. I've copied all your info and put it in a word document.

So, are the procedures pretty much the same for a water borne lacquer? I'll be spraying the new Target EM6000 in the next few weeks and I want to be as prepared as I can. I'll be shooting on practice material first, just to set up the gun properly.

Any additional help would be great, especially on reducing orange peel (I seem to get lots of it)

[Allen]

I've never used water based lacquer, so can't speak with any authority about techniques that might be specific to it.. My guess is that it's going to be very similar to solvent based ones.

Orange peel can be caused by several things.

If your air pressure is correct at the gun then the first that comes to mind is that you're sprayable product is just too thick. The finish isn't getting atomized enough and ends up hitting the surface in large globules rather than fine particles. Being too thick, it doesn't want to lay down, and you get orange peel. Try thinning the sprayable material more to see if this corrects it.

Another thing that will cause orange peel is the product being too cold. Whether you've thinned the paint out correctly or not, cold paint is always more viscous than warm paint. Cold paint is always a possibility in Canada.

When I lived in Powell River, during the winter months I had to have a hot water bath that I would sit my paint, solvents and hardeners in to bring them up to at least 20 degrees before I would spray them. Hotter was better, as the air coming out of your compressor is going to be very cold at this time of year, and will quickly chill the paint. This is also something you need to keep in mind about your spraying environment. Warm it up, as well as the thing you are wanting to spray.

Too low air pressure will cause orange peel. Same reason as the product being too thick. The product isn't getting atomized enough.

Too high air pressure will blast the paint to pieces and dry the solvents out before they've had a chance to hit the surface, not allowing the finish to flow.

The last thing that I can think of right now just comes down to spray technique. When you spray, try to have the light reflecting off the surface in such a way that you can see the build that you are applying. Never watch the gun, as you should be confident in your technique. Watch the surface and try for a uniform thin wet surface.

[Rod True]

Thanks Alan,

Upon review, I'm sure it's all of the above


NO, not really.

I am spraying 2# shellac which I'm sure is less viscous than I remember (it's been a few years since I used my spray equipment).

So, I'm sure that I didn't have the pressure at the gun right (I do have a regulator at the gun) and that I didn't have the spreader valve set right.

I'm pretty sure the material temperature is alright (could be hotter I guess) as my shop is always 23*C as it's well insulated and heated. Now I run my air line from my compressor over my electric baseboard heater so my guess is that the air to the gun (after the water trap) is about 30*.

I think I was blasting the surface and not having the spreader set right just bounced the material off the guitar body.

These are seal coats before spraying the WB so I don't mind doing some sanding and getting the "feel" back and learning what the settings on the gun do.

I'll be sure to spray on a test board from now on. Darker wood is better to set up the gun on and test yes?

[Allen]

Unless there is some specific reason to have the spreader valve turned in, then it should be set at wide open for most applications.

The fluild control valve should be set on wide open as well, until you've done some test spraying to see what's happening. If your air pressure is right, sprayable material is the right viscosity, and everything is nice and warm, and the finish is still orange peely, then turn the fluid control valve in 1 full turn, then try agian to see if there is an improvement. I've never had to go more than 2 full turns.

The fluid control valve is a fine tuning of the amount of fluid going through the gin. If you are still having problems at 2 full turns in, then you more than likely need a smaller fluid tip, needle, air cap combo.

I've never sprayed shellac, so don't know about its spraying properties. I know that alcohol is very volatile, so it's going to dry out of the atomized mix very quickly. You will want to drop your air pressure when spraying it. Don't know by how much, you'll have to do some spray tests to decide. If the edges of the spray patten on your test panel look like they are dry and powdery, then drop the pressure.

It's easier to see the reflection of the finish on darker colors. You can use something that is smooth and hard to test on. It doesn't need to be wood. You are just trying to get a feel for the gun, the product, and the conditions. I always get the apprentices to spray out on old bonnet's (hood for the Canucks in the crowd). We have heaps lying around and it's good practice for them to cover the whole surface in a nice even flowing coat. You could do the same thing with just water in your gun for some practice.

[walcen]

This is probably a dumb question but here goes.

Can a suction feed spray gun be converted to also use a gravity feed pot?

I ask this because I've been given what appears to be a reasonable quality spray gun that looks like it was designed to be made to work either way as it has a round molded section to one side of the nozzle where one could possibly drill and tap in a fitting.

Or would it be a waste of time and effort?

regards to all,

Wal

[rod]

Wal,
I think it would be a waste of time! I do a lot of spraying at work doing the galley's in motorhomes,timber moulding etc etc, and myself and family have always owned smash repair business's. Over the years I think I have tried them all,but the one that impesses me the most for spraying guitars is a Star gravity feed touch up gun similar to the one Allen has. The new model has a air control, which is situated under the grip,fan adjustment on the side and flow control on the back of the grip.I have recently purchased the new model, and surprised to pay only $85.00.
Anyone interested let me know and I will see if I can get them at a better price...
Cheers,Rod

[Allen]

If the gun sprays fine as a suction feed, then I wouldn't bother with trying to convert it. In fact, the ordeal of trying to make it into a gravity feed with unknown result would put me off of it.

[walcen]

Wal,
I think it would be a waste of time! I do a lot of spraying at work doing the galley's in motorhomes,timber moulding etc etc, and myself and family have always owned smash repair business's. Over the years I think I have tried them all,but the one that impesses me the most for spraying guitars is a Star gravity feed touch up gun similar to the one Allen has. The new model has a air control, which is situated under the grip,fan adjustment on the side and flow control on the back of the grip.I have recently purchased the new model, and surprised to pay only $85.00.
Anyone interested let me know and I will see if I can get them at a better price...
Cheers,Rod

Thanks so much for the reply guys, I'll leave it as it is.

yes Rod I have used Star airbrushes and they are very good as well.

I'm very interested in that touch up gun that you mentioned and if you wouldn't mind giving me the model number I'll see if I can chase one up for myself.

regards

Wal

[John Maddison]

I have recently purchased the new model, and surprised to pay only $85.00. Anyone interested let me know and I will see if I can get them at a better price ...

Hey Rod ... I'd be in on a group buy if & when it happens. Or maybe just point us to your supplier and we can purchase individually to save you any hassle. Cheers & thanks!

[liam_fnq]

Allen (and others), have you got an opinion on Star sprayguns. I wouldn't know one end of the spraygun market from the other. I'm sure I'm not the only one .

[Allen]

I've only used them for a cheap primer gun. The 2 pac primer and putties we use are brutal on spray guns and kills them in 6 months no matter how much money you spend. The ones we get have at work have a nozzle like a canon but they will have finer ones on their other guns.

Pretty much any reasonably built gun can be set up for spaying lacquer. I'll help you get it spraying right if you get one.

[Nick]

Liam, I've used (& still have) Star guns, I would probably only tend to use them more for the finish coats where any inconsistency can be sanded/flattened out. They do a more than adequate job for the $ compared to spending big bucks on a DeVillbiss gun (Which is more of a 'lifetime' gun as mine is and can be handed down between generations). I wouldn't use them for finer jobs such as sunbursting incase of the chance of uneven coverage & the chance of spitting.