[wvguy]

99% of all garage paint is only rated for concrete. Wood expands and contracts way too much to put a floor paint on it. Remember you can use a 100% acrlyic latex stain. As others have said cleaning is important. remove all the loose paint you can then use two coats of solid latex deck stain. Also, some paint and stain companies have different recommendations for there product. Example some use a primer(Behr, Cabot, etc.) and others are self priming(Olympic etc) Either way they all will request all loose be removed and a good deck cleaner used to remove dirt as well as kill mildew and mold. This might be your most economical route.