I just did a little of experimentation of one practice door, and here is what I found.
Parameters:
Air: 68F
Hand sanded one side, Ro sanded the other, 320 grit for both
Two new foam brushes, 1" Jen
Strained Schooner 96 into both, into small plastic cups, and covered, let sit 5 minutes.
Cup 1 I thinned 25% or so with 333 brushing liquid.
Cup 2 I thinned 25-30% with Mineral spirits.
I let both cups rest for 5 minutes after a gentle swirling motion.
Findings:
The cup with the 333 thinner actually flowed out nicely, very very few bubbles, like glass. I tried a gentle hair dryer after a minute, and that made things a little worse, and it skinned over, don't do....
The cup with the mineral spirits flowed well, but a couple of bubbles appeared, not many but more than the 333.
Additional suggestion, to me and others that I noticed. Do not press the foam brush into the bottom of the container, as I did the first time, bubbles came out of the foam...bad news... The 333 thinner is a thinner for higher ambient temps, and REALLY slows the curing process when used in cold temps, as it is still tacky for a long while..I have a clean room which helps.