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A few days ago, I sprayed a guitar & for the first time, ended up with really bad blush. (This was the sealer coat, Mohawk Vinyl Sealer)
I have had a hint of white in the past, but it always cleared up after a day in the dry room.
This time, it was awful. Snow white everywhere except where there was a brace or a block glued to the inside.
The temp was in the mid 70s & the RH was in the high 70s when I sprayed.
I had drained the tank, filter & down pipe & purged the system before I started spraying.
Still, I have a feeling that the biggest problem is with moisture in the lines.
In a recent thread, a few folks mentioned a dryer in the system. Since there are several types available, I would like to know what kind of dryer you folks are using. I've been checking out the dessicant units as they are the most reasonably priced.
I have a good DeVilbiss water separator / filter in the line.
A tech I spoke to this morning suggested a "toilet roll" final filter. Anyone have experience with these?
I am planning to use a disposable "bulb" filter at the gun as well.
Any suggestions will be gratefully received.
Thanks. Dan

Perhaps RH in the high 70's is a factor?

I'm sure it IS a factor, but I have sprayed successfully at high RH in the past. Usually, when the temp was lower though.
If I have this right, the absolute humidity will be much greater at higher temps, so there is more moisture available to mess up the finish. So... the ambient air may be the main issue.
I hooked up my gun and blew some air into my hand today. The air felt a little moist, leading me to my (perhaps faulty) conclusion.

Daniel, my bet is also that atmospheric conditions are more likely than moisture in the lines (assuming you're doing the obvious things to drain and trap water). What you probably felt when you sprayed your hand was the cooling effect of pressurised air. This cools the liquid being atomised by the sprayer. If the droplet temperature falls below the "dewpoint", moisture condenses on the droplets and is drawn into the finish. Dew point is actually a function of temperature, humidity and atmospheric pressure -- see also my reply to Lars:
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=28196&p=377216#p377216
At 70% humidity, you're certainly approaching a danger zone -- the old 60:60 rule-of-thumb used to be that you never spray unless the conditions are "above 60°F and RH below 60%". If I followed that, I'd be hard pressed to spray where I live; instead, I now require a dew point in my spray booth at least 6C° (note: C as in "celsius") below ambient.

Don't know about the sealer product you're using, but for most lacquers, you needn't do anything but wait for better conditions and recoat -- the blush will simply disappear. Beware of using ultra-slow thinners, retarders, or blush "preventers", as these can adversely affect your product cure -- read the data sheets from the manufacturer.

I'm afraid you may be right, Tim... It'll be weeks before the weather changes much, here.
Guess the customer's just gonna have to wait.
I did open up my filter & took the ends off the hose this afternoon. Both were quite wet on the inside. Lesson learned about more thorough maintenance in humid conditions.

I use a dessicant dryer on my system.

You could sand the clouded finish off, since it is only a coat thick. Then respray it with a bit of retarder in the mix.

Did you thin the vinyl sealer before you sprayed it? If not, that might have been the problem.

It's happened to me many times-spray a "retarded" mixture and it usually goes away.

Higher temps, especially combined with fairly high RH will cause more water in your air lines. I've got one of these filters. http://www.tcpglobal.com/spraygundepot/itemdetail.aspx?itemno=DEV+HAR-602 Which isn't expensive compared to alot of units. I also use one of the little throw away filters at my gun.

_________________
I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Hi Dan,

As Brad said, a "retardant" in lacquer will give a longer time before the 'skin' develops on the lacquer, and the extra time will (hopefully) be enough time to allow the water to flash off. I also recall spaying a thin coat of pure lacquer thinner on some cabinet doors one time, to re-dissolve the film and allow the water to get out.

Google turned this up, and it might provide some useful hints: http://www.woodweb.com/knowledge_base/C ... shing.html

Dennis
p.s. I sprayed water-borne 'lacquer' one time in extreme RH, got heavy blush as I sprayed, and all of the blush disappeared on its own. It stuck in my mind as one huge advantage of water-borne.

{edit} One more: As others mention, I use 2 oil/water traps: one at the air outlet of the compressor, and one at the gun. On a really humid day, bleeding the tank before starting may not help, because you're pulling humid air right back in and compressing it.

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Thanks for the input, guys. I did leave the guitar in my dry room for quite a while & there was no change. Since it was only one sealer coat, it was easy to scrape off.
I had thinned the vinyl sealer about 15%.
Between the moisture in the lines & the high ambient RH, it was a losing battle, for sure.
I will check out a dessicant dryer. It looks like another water trap & down pipe at the compressor might help too.
Cheers Y'all.
Dan

Bite the bullet and buy a refrigerant dryer and never have oil or water in the lines again. I picked one up at either Harbor Freight or Northern Tool for $299.99. Best money I ever spent on a spray system.

That first coat blushed because of humidity. If it was water in the lines there would be little craters in the finish sort of like fish eyes that are caused by oil contamination but much smaller. The next coat applied in dryer conditions will burn it away.

If it is humid and I need to stay on schedule I add a splash of retarder.(Read that a cap full) Once retarder is in the mix you can not apply as thick a coat because it will run easily.

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"Try to be the person your dog thinks you are."

http://sites.google.com/site/bertsguitarshop/

1) Did the white stuff look smooth or fuzzy on the surface?

2) What did you thin with?

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Howard;
I thinned with lacquer thinners. About 10%.
In the worst areas, the surface DID have a kind of "fuzzy" appearance. It didn't feel rough, though.

Hardware store thinner? If the appearance is fuzzy, you may have "cotton blush." This is not caused by humidity. It is caused by cheap thinner that contains too much vehicle (the toluene and xylene, which are cheap) and not enough solvent (alcohol, acetone, butyl cellosolve, etc., which are more expensive). What happens is the nitrocellulose comes out of solution and turns to cottony white stuff before the vehicle is all evaporated. It's supposed to work the other way around--the finish flows out and the vehicle is all gone before the nitrocellulose comes out of solution.

I stick with proprietary (e.g., Behlen, Mohawk, etc.) thinner for thinning the lacquer and use the hardware store stuff only for cleanup. It actually is not all the same stuff, and it matters.

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Good advice Howard, but I only use generic lacquer thinners to clean the guns. The only thinners in the spray room is Mohawk brand. Except for the freight costs, it's actually less expensive than the hardware store stuff.