Most of My Finish Problems Started With a Dirty Spray Gun
One of the biggest mistakes I made when I was first learning finishing techniques with an HVLP spray gun was not cleaning it well enough. I’d clean the gun, think it was clean, and it wasn’t. Then the next time I used it, I’d get stripes in my finish, and all kinds of other problems would show up. Looking back, I’d say ninety percent of the finishing problems I ran into came down to one thing: the gun wasn’t as clean as I thought it was.
Through a lot of trial and error, I’ve worked out a cleaning process that gets the gun perfectly clean for the next use without wasting a bunch of thinner. Here’s how I do it.
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If I’m Coming Back in a Few Minutes for Another Coat
If I’m still in the middle of a job and I know I’ll be back in the spray booth in the next 30 minutes or so for a second coat, I don’t break the whole thing down. I just clean the tip.
I pull the air cap off and drop it in a small plastic container of thinner to soak while the finish dries on the piece. I’ll also wipe a little thinner on the tip itself to knock off any residue. About 90% of the time, that’s enough to keep the gun clean and ready for the next coat. When I come back, I just put the air cap back on, and I’m spraying again.
If it’s the last coat of the day, that’s when I do the full breakdown.
Saving the Leftover Finish
Once I’m done spraying for the day, I pour any leftover finish into a secondary container rather than back into the original can. By that point, I’ve usually added retarder, thinner, and sometimes a catalyst depending on what I’m spraying. Pouring it back into the original container can contaminate the whole can. I do hang onto that leftover finish until the next day, once the parts are dry and I’ve had a chance to inspect them for spots that need a little touch-up.
The First Rinse
I disconnected the air hose, and I’ve still got that cup of thinner I was soaking the air cap in between coats. Since it’s already dirty, that becomes my first wash. I pour it into the gun’s lid and work it around with my gloved finger, working it into the threads, then pour it into the gun body itself.
From there I take a paper towel, dip it into the thinner, and wipe down the interior walls of the gun, working my way all the way around the cup. Then I go back to the rim and scrub the threads. Any junk that dries in the threads can end up inside the gun and plug up the tip. Dried finish also makes it harder to get the lid back on, so I take my time here.
At this point, if you look down into the gun, that first rinse of thinner is pretty murky. There’s a lot of residue mixed in with it.
Flushing the Spray Gun
Next, I hook the air hose back up and flush the gun, spraying that contaminated thinner through it to clear out the needle, the cap set, and the body of the gun. It’s not much liquid, just a lid’s worth, so I’m not worried about where it lands. It evaporates fast.
The Second Rinse
Now I do it all again, but with fresh, clean thinner. I fill the cap, use my finger to clean the edges and the inside of the cap, pour it into the gun body, and go over the walls again with a fresh paper towel to make sure there’s no residue left that could dry and flake off later and plug the gun. Then I clean the threads again.
This time, when I look down into the gun, the thinner comes out almost clear. That tells me we’ve done a good job getting the residue out. I hook the hose back up and spray that thinner through the gun one more time. If the thinner was cloudy, I would repeat this step a third time, but usually twice is all I need.
Breaking Down the Small Parts
With the body of the gun flushed out and clean, I pull the air cap off and drop it in the cup of thinner to soak, along with the needle cap that holds the needle in place, and the tip of the needle itself. Depending on your gun, you might have other parts in there too. Mine has an air diverter, so I hook up the air hose and give it a quick blast to knock that loose, then it goes in the thinner cup as well.
The last thing I do to the gun body itself is take a small brush and scrub out the tip and the cup, both orifices, giving them a good scrub. Then I pour a little more clean thinner through the gun for one last rinse. At that point, the body is totally clean and ready to set aside for next time.
Cleaning the Small Parts
Now it’s just a matter of cleaning up the little pieces. I start with the air diverter, wiping off any leftover finish or thinner. Then I use cleaning needles that are specific to that air cap.
I fish the tip out of the thinner and use the cleaning needle to clear out the center hole. Then I pull the air cap out. It’s got three holes on either side of the main hole where the needle passes through, plus two adjacent holes that divert the air and shape the fan pattern. I go through and poke each one of those holes with the cleaning needles to make sure there’s no debris or obstruction, then drop the cap back in the thinner to soak overnight.
If I know I’m spraying the next day again, I let it soak overnight, and I’m ready to go in the morning. If it’s going to be a week or more before I’m back in the gun, I only let it soak for about an hour, long enough to get the residue off, then pull the needles and cap out and let them sit and dry.
One Last Habit
Once the gun’s clean and ready to go, the last thing I do is lay out the paper towels I used to wipe it down so any leftover finish or thinner on them can evaporate safely, rather than balling them up somewhere. I’ll be honest, the finish I’m usually spraying isn’t the kind that’s prone to spontaneous combustion the way an oil varnish is. But I think we’re creatures of habit, and if we build good habits, we’re a lot less likely to burn the shop down on the day we forget which finish we used. So I lay everything out every single time, whether it’s strictly necessary or not.
What It Costs
When it’s all said and done, this whole process uses about three lids’ worth of thinner and three paper towels. That’s not a lot of waste, not a lot of cost, and it only takes a few minutes. Compare that to the alternative: fighting contamination and poor finish quality in your piece because the gun wasn’t actually as clean as you thought it was.
I’m the owner of Benham Design Concepts, a mixed media art studio where I design and build custom furniture and other works of art using wood, glass, stone, and various metals.
In this blog, I talk about the art I create, my journey, and the things I learn along the way.