problems spraying poly

[Paul Henneberry]

Hi,
I've switched to polyurethane from previously spraying nitro and am experiencing problems with little craters all over the surface and am looking for some advice on a probable cause. The poly I settled on using is Northane made by Norglass which is an Aussie made two pack. I picked this because it is available locally and there isn't much to choose from over here in the west. I could get mirotone but the distributor is miles away. My instrument assembly method for my ukes is to fully finish and polish the soundboards before attaching the necks and that is where I'm up to. When I was spraying nitro I used to grain fill with cyanoacrylate but wasn't sure about the compatibility with poly so I thought I'd try out poly's legendary non shrinkage and go straight onto the timber. I've given the soundboards two initial light coats an hour apart followed by a days break, then a levelling sand followed by two wet coats followed by a days break, then a levelling sand and I just sprayed another wet coat and I'm still getting craters. Northane stays sticky for about an hour and builds really thick so I assume it has run into and filled and sealed the grain by now. I don't have any silicon in the workshop so doubt that is the problem although it can't be discounted. Four of the five ukes in this batch are made from a 175 year old mahogany dining table that I picked up so maybe there is silicon. The fifth uke is spruce and the soundboard was sanded using different thickenessor belts so I don't think there was any cross contamination but I'm still getting cratering on this one as well although not as bad.


Here's what I think might be causing it but I don't know much about this stuff and am open to some expert advice. I picked up cheap a new Norgren air regulator and drier/filter pair and recently fitted them to my compressor. High quality stuff, looks good. I also got one of those very nice 3M 75mm pneumatic sanders that Allen McFarlane was using in a video on fine wet sanding last year. Brilliant little tool by the way, thanks Allen. This sander uses a lot of air and I have been using it a fair bit. I started to notice some water droplets being blown out of my air blast gun which had never happened before. The filter element in the new air drier was very damp but there wasn't any water in the bottom of the bowl? When I checked the filter model number on the Norgren site it is fact a special oil coalescer separator. These things have a flow path opposite to what you would normally expect for an air filter. So yesterday I swapped out the oil coalescer for a normal water separator/ filter. I then emptied a tank of air through the regulator / filter and hose in an attempt to dry it all down. I just mixed up some poly and laid down another coat but I'm still getting cratering. If I'm onto something with water contamination and this water is in the hose how can I dry out the hose?


I'm probably on a wild goose chase with the water and would appreciate some other opinions about what might be going on.


cheers


Paul

[demonx]

Without seeing it in person or without even seeing photos it’s hard to imagine what you mean by crators, are you talking about solvent pops?

It could be anything from contamination to bad mixing (stir minimum 100x alternating left and right every 20x) to poor spraying technique.

In my acacia build thread which is a couple posts below this one I typed out a lot of hints on the 2k spray process, it may be worth a read to start off with.

[Allen]

Hard to say Paul, but never in a million years would I attempt or recommend to fill pores with the finish as you are describing. Even without getting the results you are experiencing, the build of material you are getting is going to be way too much for an instrument.

[Allen]

If you are getting craters on a spruce soundboard, and they look similar to the open grain hardwood, then it's not the pore filling that's causing this.

When you sand back and check very carefully, are there any craters left?

If not and then they appear in the next coat, are they appearing in the same spots as you last sanded them out, or randomly somewhere else?

If random and they are also showing up on the spruce soundboard then my guess is down to 2 things.

First up is you have to have dry clean air. No secret that moisture and oil is going to give you grief.

If you are 100% sure that the air (and hose if it was contaminated) is good, then the next thing I'd say is solvent boil.

You tell the difference between contamination or solvent boil by observing how the craters show up. If after you spray the wet flowing coat the surface is nice and smooth for at least a couple to severall minutes then it's not contamination. Sovent boil will show up as fine pimples say 1/2 hour to several hours later depending on temp. and humidity. If solvent boil is severe then those pimples will burst and cause craters. Sometimes they may not burst, but will be trapped inside. Offten you can observe this condition on a "fat edge" where the finish gets a little to thick and you can see many very fine white dots in the finish.

[Paul Henneberry]

Thanks Allan and Allen,


I wrote everything below early this morning and have then spent most of the day trying to attach some pictures. I've done it before without problem but today when the little status green bar for uploading a picture gets to about 90% it turns into a little yellow triangle? I've emailed the administrator and for some help.

I'll add pictures in the future but I am looking at maybe 50 randomly spread shallow flat bottom craters ranging from about 1mm to 3mm in diameter. The rest of the surface is almost glass smooth. They appear on the wet surface within a minute of spraying. Significantly, they weren't as bad on the spruce SB on the first coats but there are more of them on subsequent coats. I think they appear in different numbers and locations in successive coats. I ran my air line out through the door and left in the sun with air trickly through it in an attempt to liberate any moisture that might be stuck to the walls.


Now this all out of order but below is what I wrote early this morning.

A bit more background, I'm spraying with a little Star miniSV-106T with a 1.2 tip. A bit small but I make do, it worked fine with the nitro. This is a gravity HVLP gun and I'm spraying at 200kPa. I'm using the poly manufacturer thinners (Northane - spraying thinners) at 33% as per there recommendation. Probably not mixing as well as Allan recommends - maybe the problem. Following the manufacturers recommendation of mixing A and B then leave for 10 minutes before adding the thinners and then waiting another 10 minutes before spraying. About 50ml has given two coats on the 5 soundboards so I'm mixing based on weight on a 0.1 gram scale. I got the densities of the three components from the manufacturer and corrected for the volumetric mix ratio. I add the liquids to a clean mix pot on the scales using clean syringes. Dedusting the sound boards with compressed air prior to spraying. The weather has been mild in Perth so probably about 25degC when I have been spraying. Humidity has been less than 30%. I sanded away most of the pair of initial light coats to level the finish and exposed wood in a couple of patches so it was only really pore filling.

Allen, what do you grain fill and seal with under poly? Is cyanoacrylate OK 'cos I was happy with the results under nitro when I was using Starbond EM150. What about mixing an egg cup full of 2K poly and rubbing it into the surface as a filler. It might be better than CA because I would have longer to work it before it thickens up? And it seems to sand better than epoxy.
I was approaching this poly application process like I did with nitro in that I was going to lay it on a little bit thick with the expectation of having to fine sand away a fair bit of the surface to remove any imperfections (it’s only a hobby, I have the time) prior to buffing but it’s looking like this is a very different material that needs a different approach. I’m just spraying in the corner of the workshop so it's never going to be perfect off the gun especially with this much longer tack time.
cheers
Paul
Postscript : I would have posted this sooner but was having problems attaching pictures. 16 hours after spraying curiosity got the better of me and I attacked the soundboards with some 320 Velcro drylube in a random orbit sander. I quickly got rid of 90% of the craters without going down to wood. It looks like this is recoverable but what caused the blemishes?

[kiwigeo]

I wrote everything below early this morning and have then spent most of the day trying to attach some pictures. I've done it before without problem but today when the little status green bar for uploading a picture gets to about 90% it turns into a little yellow triangle? I've emailed the administrator and for some help.

Paul, check file size. Alternately your files may be corrupt. See PM.

[Allen]

I use Bote-Cote epoxy for my pore filling. I can get it locally, but it's so good I would happily order direct and have it shipped. They have a solvent called TPRDA to add to it for deeper penetration and this works a treat.

That size of crater is definately not solvent boiling. It's contamination of some sort. Oil in the air coming from the compressor would be my first guess. This can be a complete bastard to get rid of. When we had piston driven compressors go bad in the paint shop it meant that we fixed that (rebuild the head), and then replaced all filters and hoses to get rid of the problem. Even then there was oil still in the hard plumbed lines that would take some time to work its way out of the system.

[Paul Henneberry]

Thanks Allen.
I did check the oil sight glass the other and it hadn't used any oil. Since I corrected the problem with the wrong type of filter I haven,t seen any water droplets coming from the blow gun. I used a fair bit of air yesterday on the sander and what with warming up the hose in the sun maybe I've expelled the last of the moisture is that is what it was. I can now aim the blow gun at close range with the trigger depressed at a bit of mdf for a few minutes without any type of moisture showing up.
Thanks again Allen
And thanks Allan, I did read your whole build post and it was very informative.
Cheers
Paul