Troubleshooting Small Bumps in Finish

Here's a long list of suggestions for what might be causing small bumps to appear in a finish coat. April 18, 2015

Question (WOODWEB Member) :
I can't seem to diagnose a problem. I'm using a gravity fed cup gun and spraying pre-cat lacquer. This is on cabinet doors in a clean environment. I vacuum the doors after initial seal coating and sanding. I clean the gun. I get small bumps and can't tell if they're shooting out of the gun or circulating dust somehow. I use filters when I pour my lacquer into the cup. These bumps sand down and go away but new bumps appear in the new coat. Too much pressure, not enough pressure?

Forum Responses
(Finishing Forum)
From Contributor S:
I have found that if you don't break down the gun periodically and really do a thorough cleaning you end up with what you describe. Just running a bunch of thinner through the gun doesn't get all the finish out of the junction area where the cup screws into the gun. Over time the finish thickens and comes out of the gun in small globs.

From Contributor C:
What is the ambient air temperature? Is the spray drying before it hits the surface? I have had this happen during the hot season. I believe the solution is to add a retarder or flow enhancer. Check with your finish supplier to learn what works best for the pre-cat lacquer product you are using.

From Contributor R:
Are you using a fine mesh filter? The ones sold at auto body shops that are meant to be used for color coats work very well. They have a small closed area at the very bottom of them and it prevents any clumps from getting into your coating. Perhaps the bumps are flattening paste that wasn't dispersed into the coating as you were stirring.

From contributor C:
What kind of moisture filter do you have and how far away from the compressor do you have it mounted. This time of year can be some big problems with condensation in the line. Is it really a bump, or crater?

From contributor X:
Are you getting the bumps on just pieces you sprayed flat? If so, it's probably dirt in the air. If you are getting bumps on everything (vertical and horizontal), then it's coming from the finish. If you can determine one or the other that will prevent you from chasing your tail around.

From the original questioner:
Thanks for the answers. My hose is 50 feet. I have a nice compressor with dryers and water traps and then I have a disposable trap at the gun. I only spray horizontal surfaces. The bumps are not craters. The lacquer takes a few minutes to heal over. I am using a retarder also. The temp is 70-ish. My best guess is that clumps are building up in the gun. Are there guns that are "professional" gravity fed guns? Or is that a contradiction. One thing is that the bumps many times are more frequent in corners, like a shaker door at the flat panel. I vacuum out the doors with a Festool vacuum.

From contributor K:
My SATA gravity feed was $750. I would certainly hope it would be considered a "professional" spray gun.

From contributor C:
Are the dryer and water traps mounted on the compressor? It needs to be at the end of that 50' hose. Water traps at the compressor is useless. The air temp coming out of the compressor is too high to let the water fall out of the compressed air. I always used a desiccant filter at the gun when the humidity went high. Can't say moisture is your trouble, but it can be.

From contributor G:
How old is the product? I got a 20 liter of post cat product that did that and it ended up being old product. The supplier called it seedy. I called it a waste of two days! I would try a fresh bucket if you have one available.

From contributor S:
Like Contributor K said the Sata Gravity fed guns are probably the best and most popular guns, more so used in the auto painting world. For spraying lacquer for wood finishing the 100 model is sufficient and they run about $350-400. Like others said even a lower end gun won't cause your problems unless it is dirty. If it is fish eyes or little moon like craters it is caused by contamination in your airline, compressor or even some blow guns have silicone in them. If it is debris in your finish it is caused by either something in the air or something in your material. Debris in your material could come from a dirty gun. Clean your gun good and strain your material and then you can narrow down the cause.

From contributor X:
Since it is more frequent in the corners of a shaker style door I think the problem is sanding prior to the top coat. It's fairly common for people to not sand completely in the corner of those type of door styles. If there were a few bumps that went unsanded they will look worse after the top coat because you’re just building up the bumps even more. If you are certain they are thoroughly sanded then my next guess would be using a Festool vacuum to remove the dust. We also use a vacuum to remove sanding dust, but we also blow them off with compressed air prior to applying the top coat. That gets the rest of the dust and dirt out the vacuum leaves behind. I think if it was something in the finish it would be equally bumpy everywhere.