Leaving lacquer in spray gun
Will a few hours between coats gum up the gun? September 2, 2002
Q.
Can I leave lacquer in the spray gun for an hour or so between spraying coats, or will it harden enough to gum up the gun? I will be spraying Valspar Valtec precat.
Also, what should be used to slow the Valtec down if I need to? Is there a humidity limit that should not be passed (I have heard 70% relative humidity)? Lastly, is it necessary to scuff sand between coats (with like 320 grit) with the Valtec, or do you only have to sand if there is debris in the finish?
Forum Responses
1) Many people do. I don't. As easy as gravity feed guns are to clean, I just pour the excess into a mason jar. Then I pour some thinner into the cup spray for five seconds and let that sit after swishing around. The whole process takes less than a minute.
2) MAK (methyl-amyl-ketone)
3) 70% sounds right. Since I live in a desert, the humidity isn't this high when it's raining.
4) Scuffing is always a good idea.
It's no problem to leave the lacquer in the gun for a couple of hours. Just give a quick shake to remix the flattening paste back into solution.
If you don't want to sand with paper, you can use scotch brite pads to take the sheen off, which is usually enough for proper adhesion between coats.
The 3/16" very fine sanding sponges are fabulous, as well. These last forever and can be easily blown out with compressed air.
Are these sanding sponges 3M products? I have never used them but am sure open to a better way to sand sealer. Do they work for sanding between finish coats?
3M makes them as do others. The good ones are all actually produced in the UK and distributed in the US by 3M, Norton and others. The ones you want are the ones made on open cell foam like the Norton. These you can blow out again and again and they last forever. The Klingspor are not nearly as good as they are closed cell foam and they don't clean as easily. They also are not as flexible as the open cell. These thin sponges are simply great for between-coat scuffing. They level better than Scotchbrite and leave a more uniform scratch pattern as well. Use them for all coats. You'll like them.
My gun always has lacquer in it. I spray a little thinner through it every once in awhile. Sanding sponges rule - they last longer, they are easy to hold onto and much easier on the hands.
I also leave precat or NC in guns during the day and overnight if the wetted parts of the gun are stainless. Any aluminum parts will cause some problems on acid cured finishes.
As for slowing it down, Valspar likes to use PM Acetate.
Bob Niemeyer, forum technical advisor
I use Devilbis teflon lined cups on my cup guns. These are great - they wash right out no matter what you put in there.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor J:
I will leave lacquer thinner in my spray gun for hours and haven’t had any problems with my gun. It’s good for keeping it clean of products. I just give it a good shake and run some air through it and it’s clean and ready to use.
