I've been spraying some good old fashioned nitrocellulose lacquer onto some small delicate pieces, and I keep coming up with fat edges. I have extra finish that collects around the edges adjacent to a shaped edge. Initially on pieces prayed vertically, but it persisted even after I switched to spraying the pieces laying flat.
I've sprayed a lot of pre-cat onto larger pieces both vertical and horizontal and any fat edge was the result of a bit too much product on a vertical surface. In other words there was a little sag. It’s easy to remedy, but it's proven to be stubborn with the nitro. What am I missing?
For reference the nitro is being sprayed over a vinyl sealer. I started with an HVLP gun with a 1.3mm tip and a pressure pot and switched to a HVLP gravity feed with a 1mm tip. My viscosity is fine and the problem seems to only occur around the transition from a flat surface to a shaped edge. I haven't seen the fat edges on any other surface.
From contributor C:
You’re saying that when spraying the pieces laying horizontally (flat) your getting a lip on the bottom edge of the pieces? If so try backing off on the fluid rate (restrict the flow) a little and see how much improvement that gives you. That's all I have ever had to do in the past to remedy the problem.
What can be significant is the distribution of electrical energy (EE) over the surface of your piece. EE doesn't move around much on an insulator like wood and it likes to hang around sharp edges. This concentration cause areas of higher critical surface tension which in turn causes your coating to flow out more in these areas than in the general field of your piece. This can really be a problem when air is very dry.
A couple of suggestions:
1. Wipe your piece down with a solvent that has a very low electrical resistance. This will allow help redistribute the EE more evenly. Acetone, MEK or ethanol would work. Spray it as soon as it's nearly dry.
2. Add a wetting agent or surfactant to your finish. This will lower the finish's surface tension. The farther below the critical surface tension(s) of your piece you can get the finish surface tension, the more consistent your flow-out will be.
3. Spray thin coats of thin finish. The thinner you can keep the coat, the less chance it has to move around. If you need to thin, thin with a very fast thinner.
I know what I'm seeing is partly surface tension, but I won't be able to experiment with eased edges until I make new parts. You got me again though, as I have no idea what a wetting agent for nitro lacquer would be.
I've suspected all along that thinner coats of thin finish is what I need, but I haven't hit the right combo for that as yet either. If I spray what I consider a wet coat the fat edges appear. I haven't hit the right viscosity yet to allow a very thin coat to flow out. On top of that, in my mind the problem is that the lacquer is drying too fast, not too slow.
Should I shoot a light, sparse mottled coat and wait for it to flow out? Is that the idea? I've been shooting light coats but with thin product tonight and it seems to help but not eliminate my problems completely.
Should I be thinning with the B.S. only? I've shot many a cabinet, door and piece of furniture with the pre-cats without a fraction of this problem. These pieces are small and that must be a factor.
There's no reason the NC lacquer should be very different from the pre-cat lacquer if the solids content, viscosity, and spray gun settings are all similar.
Fat edge is simple.The finish is being applied too thick. The no brainer solution is to add thinner in 5% increments until you can spray a normal pattern and have it go away. The trade-off is that the more thinner you add, the thinner you'll have to apply each coat to avoid sags and runs. And it will take more coats to build the same dry film thickness as you would normally achieve with finish that's thinned less.
I thinned it until a had 19-20 seconds through my cup, #4 Ford which I've tested against my supplier's Zahn cup for comparison. This is what I usually do when I thin.
It was too thick plus the sharp edges are surely the culprits here. I confess I thinned a small batch pretty heavily (50-50) and added some BC and didn't even touch my Ford cup. Between the thinner coats and a simple change of pattern in application I finally managed to eliminate the fat edges all together. These pieces are app. 9" x 12" and 3/8" thick, so the light thin coat works fine.
I'd already resigned myself to shooting more thin coats as you mentioned. Life's full of tradeoffs, and of course, at 50-50 I've halved the solids content. I think I knew all along I'd have to thin until it all worked. I just had to work up the nerve to do it.
I'm glad you got it ironed out. You could probably get away with less than 50% thinner if you wanted to. But if it's not a finish you'll be working with very much, then there's not a lot of value in finding out the minimum percentage of thinning that works well.