I agree with kok. You want to remove the old finish first, then follow the directions in the paint kit. If you don't you will soon be as unhappy with the new paint as you were with the old. There are two problems with just covering the old paint. 1) Either the oil based paint wasn't made for garage floors or the concrete wasn't properly prepped first (or both). Therefore it will eventually flake up from the concrete. 2) Whatever is coming through the paint causing it to yellow isn't going to go away by putting more paint on top of it. If you will strip the old paint and then as kok suggested - acid etch it, then use garage epoxy, you should be happy with the results for years to come. I've painted a concrete floor before, and can tell you that proper prep is worth the effort.