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How far do I go before I stop sanding? When I no longer see the old color? I am using 50 grit right now, should I up that to a finer grit paper? This is my first time sanding anything. Thanks.

First thing, I hope you washed the car thoroughly and then used some type of wax/contaminant remover. Prep-Sol is the "correct" cleaner but I've had luck with washing with Dawn dish soap and then doing a mineral spirits wipe down. Wiping the mineral spirits on with one rag and then following up immediately behind with a clean rag to collect the nasty stuff (silicon, wax, etc) trying to rotate the clean rag to not wipe stuff back onto the surface.

I would not sand all the color off. Just sand enough to key the old paint (take the gloss off and give some fine scratches for the new paint to grab onto) and smooth it out (ie get rid of the pits and high spots) without going to metal or bare plastic. Any areas that still have depressions, pits, etc fill by using brushed primer or bondo if necessary and then re-sand the areas to smooth it back out. I would throw that 50 grit into a drawer. It is way too rough for what you need. It will leave scratch marks that you will have to refill with primer/paint to get a smooth surface. For the initial prep sanding of the old paint, I wet sand using 150-220 grit wet/dry paper.

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Regarding the ACE brand Rust-Stop oil based paint. My local ACE hardware stores have tintable bases and will mix just about any color I would want. I didn't know this before this past week. It would be nice if it was a rebranded Rustoleum, but I wouldn't count on the store workers being able to give correct information.

I'm trying the BPS (Tractor Supply) paint made by Valspar (New Holland yellow). It seems to work fairly well on the test panels I've done so far. I'm going thin on the paint (thinned with about 30% mineral spirits) and the coats (it'll take about 8 to get complete coverage). I helped a friend paint a new metal house door with the ACE Rust-Stop and used some Penetrol at about 15-20% with no mineral spirits and you could see the difference in how the paint laid down and that it increased the working time by quite a bit. You could also feel the difference in the paint on your fingers, it was more "slippery. I would still suggest thinning the mix a bit (maybe 10-20%) with mineral spirits. I plan on trying this next.