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the definition of insanity or spring time finish blues4/10
doing the same thing and expecting different results. this, seemingly, doesn't apply to finish work. i can do the same thing and get completely different results.
any thought/suggestions would be greatly appreciated.
i'm spraying acrylic urethane. using a 20:1 graco merkur pump with kremlin excite gun. enclosed, crossflow, automotive spray booth. i use a 09-134 tip and spray two coats (both in the same direction and one right after the other - like a box coat but i don't spray across the grain). we spray a variety of hardwood flat things. i've been doing this for long enough and have gotten back into finishing after i let my finisher go. he'd had these problems as well, and we've both achieved near perfect results off the gun with our set up and using the finish mixture below.
friday - temp 67*, humidity - 25% - retared, with slow thinner, my material 20-25% (20% is what the manufacturer recommends). i sprayed one piece of stained walnut, killed the fan as soon as i was done spraying. kept an eye on it and no bubbles seemed to form. left it alone for 15 minutes or so. rechecked and everything looked great. turned the fan back on to speed drying because i had other pieces to spray. piece 2 and 3 - stained sapele and natural sapele. both pieces were relatively small so i sprayed the natural piece first, set it to the back of the booth so i wouldn't overspray it and sprayed the stained piece. same mixture of finish. natural piece looked good initially but when i turned off the fan and looked over them both - the natural piece was bubbled up and the stained piece looked like it should. natural piece never laid out and needs to be reshot, stained piece turned out great. only difference was the natural piece had more time with the fan blowing across it while i sprayed the second piece. i left the fan off since i was headed home for the day.
sunday - temp - 61*, 19% humidity. sprayed a tigerwood piece that hadn't turned out from earlier in the week. i used the same mixture as above and did everything the exact same as on friday. piece showed a significant amount of bubbles after the first pass and pretty much ditto after the second pass. i turned off the fan, cleaned out the pump and revisited the piece. some bubbles laid out, but too many bubbles remained and i'm sure the piece won't turn out (i left before it skinned over).
any guesses as to what is going on? temperature and temperature of lumber and finish are pretty much the only differences between the days (hardwood aside). i tried a number of test panels earlier in the week (temperatures were close but humidity was up) and thinned/retarded less with the same unacceptable results.
would heating up the finish do any good? is there a finish temperature that i should aim for?
problems like this seem to happen when the temperatures are in fluctuation (usually in the spring and fall). most of the other times we can spray blindfolded and get near perfect results.
With that retarder in there it should level out pretty nice. Temps are low. Recommended temps for spraying are 25C (77F). Try to keep it in the 70s at least.
Leo G has it right. What do you think the surface temp is of the wood you are spraying? Got to warm up the place or change to Nitrocellulose.
how many of you have 77* in your spray operation? it sounds great in a laboratory but hard to implement with a limited equipment/building budget in the winter time. plus, things sprayed okay this winter and we keep the shop at 65* and it drops as we use the booth.
anybody heating up their material prior to spraying?
I actually find 77F to warm to get a decent flow out of a finish without loading it up with slow thinners. I prefer to keep the spray area and drying area at about 72F. And yes, I try to keep it there because I know the finish needs these temps to work properly.
I built an insulated drying room that I heat all winter long. It goes no lower then 65F so when I need to get it above 70F it responds quickly. It's not just the air you need to keep at those temps, its all the materials and the objects you are applying a finish to. You can't expect to get stellar results if your air temps are 75F but the materials and substrates you are spraying are 50F.
It takes a lot longer for objects to get fully warm then it does air.