Sorry it took me so long to start up, but plumbing problems in the home trumped fun in the garage:

Finally had the time to experiment with Rostoleum and Penetrol, rather than mineral spirits.

1st test: 3 parts paint to 1 part Penetrol. It never really got thin enough. After mixing and mixing, I decided to see how it would work (since I was testing on an old piece of sheet metal). Went on very nicely, covered very well (like 75 percent of the gray beneath it) and the bubbles y'all mentioned popped with a little encouragement.

BUT: came back after 2 hours in the semi-sun...still wet. Since it was all for science, I laid another coat on top of half if it, leaving the other untouched. Almost immediately the brush began grabbing the first coat, creating a gawdawful mess. I let it sit overnight. The next day, the untouched side was still tacky, and the two-coat side was a pure mess...totally covered, but a pure mess.

Upped the mixture to two parts paint to one Penetrol. Paint was much runniers, but still thicker than the "water" grade. Flipped the metal over and tried again. Much smoother, more bubbles and less coverage, so I'm guessing it's more what we're looking for.

Let it sit all day. At the end, it was still tacky, but very few ridges. However, it still would chip off using the fingernail test.

That was yesterday. I'll check it again this evening. Anyone got an idea? I figured a 2-1 ration would really be too thin, but it seems to still be a touch thick. Maybe I should add some mineral spirits just to make the mix proper?

But the orange looks great...once you get past my miserable rolling. I'll scan y'all some art this evening.