I did one plane like that, sort of. It was an OPP, aka an Other Person's Plane.  I started off with a goal of simply getting rid of the oxidized finish and some mediocre stenciling of their AMA number and racing numbers. Some very fine grit followed by compound did a fair job. A year or so later, a few structural problems and some lifting from oil-soaking became apparent. So I sanded as much of the original finish off as I could to get rid of weight without damage to the glass/wood. De-toxed the oil as best as I could and started over with KK primer and colors. Looked good for another year, but the oil kept coming up and lifting the finish and the wing joint was definitely loosening up. It's now a wall-hanger.

I found that the paint was definitely "hard enough" to sand. The minute you scuff the surface, you will get "the dull color."  To get the shine back, you have to work up in grit and then compound.

I don't know of any good way to sand most of the color off without going thru somewhere. The paint that is on it certainly won't all be the same thickness before you start. You could try to use the depth of color, but...you either won't ever remove much weight; you're gonna get blotchy color; or, you're gonna go thru in a bunch of places. I don't think the typical paint thickness tester will work without a metal base underneath.

So the likely advice would be to sand off as much as you can, and wherever you end up going thru....stop about 5 minutes earlier.

Dave

I'm no expert on Hobbypoxy, but here's my experience with it. Back in about 1970, I built a little mouse racer that I painted in red Hobbypoxy and sanded smooth, before "finishing with fine steel wool. It naturally had no shine. About 10 years ago, my club was invited to fly and display some models at the "Piston Power" show at the old IX center at Cleveland Hopkins airport, and I thought it might add to the table displays. When I got it out, despite its never having flown, I needed to clean forty years of grime off of it. I had always thought it to be fuel resistant. So I tried to clean it with rubbing alcohol. The finish softened and wore through almost immediately. It was sticky, until the stuff evaporated.  Since I had brought all my "stuff" from my boyhood home, I actually found the two-part cans, repainted damaged surfaces, and wet sanded and steel wooled the surface again in one night. I had Patrick and McGriff sign the wings the next day. So it's cool, with signatures of drivers whose careers started 50 years apart, but overlapped.  If I were to use the stuff again, I suppose I could paint over it with butyrate, as I have done successfully with epoxy/FG, but I obviously was wrong about the fuel resistance....and no, I had no real expectations of it's flying and standing up to the engine heat! 'just liked to build.