Oh no, I didn't mean for nitro. That stuff will rot your brain! Or a heater will ignite it and blow your garage sky high.
But for poly, wouldn't a heated garage with an exhaust fan work? I don't know. I've always sprayed in the summer.
Well, you're not going to be able to use poly like they do on the imports. That stuff is UV cured.
Most auto paints are lacquers and are very durable. Enamel works OK but takes forever and a day to cure properly.
Not sure what the discolorations are, but if you are good with stain, you could probably get them to blend fairly well. But maybe not...especially if there is something soaked into the wood.
I did one guitar with enamel. It turned out pretty good but next time I'll use an auto lacquer. Dries much faster than enamel.
EG