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Yes, you need to scuff up the existing paint for decent adhesion of the new paint. You can use a Scotch-Brite hand pad such as 3M #7447 (general purpose - maroon) or 400 grit sandpaper. You should spot-prime all areas that show any bare metal. My personal preference for that is an epoxy primer. After spot-priming the bare metal areas, I also like to apply one coat of epoxy primer over everything. That step is not mandatory but I like to start with a uniform color base for application of the topcoat. Such a step also provides a sealer to prevent any possible adverse reaction with the new paint. I don't think there would be any since your paint is old and obviously well cured, but it's always a nice precautionary step. This assumes that you do not use anything significantly coarser than 400 grit or do any body work such as plastic filler. After that type of work a heavily bodied surfacer would be appropriate after the primer, then final sanding with 400, then topcoating with the color. After you get all the old oil and grease off by whatever means available, you should do a cleaning (prior to sanding/scuffing) with a dedicated wax and grease remover. I use PPG's DX330 but there are others available. See here for detailed instructions. Repeat the same thing after the sanding/scuffing. That will provide the cleanest and best surface for the new primer/paint. There is no "base coat" needed unless you care to define the primer as a base coat. This assumes you are using a single stage topcoat and not a two stage basecoat/clearcoat combination. An acrylic enamel would be a very good choice. It is the best choice when trying to avoid the safety issues in not using proper respiratory protection when using hardener additives. Do a search in this forum on the term isos or isocyanates to get some background on this important matter. Make sure any acrylic enamel you are considering can be properly used w/o a hardener additive. Most can but there are some out there that cannot. For example, Ful-Cryl II acrylic enamel in DuPont's NASON line cannot be used w/o hardener. I am assuming here that avoiding a hardener is a prime consideration of yours. If you don't have fresh air breathing equipment, it should be. Fast-Dry acrylic enamel in the NASON line doesn't require such an additive and would be a choice, although I have never used it. So would MAE in PPG's OMNI line. The top acrylic enamels are DuPont's Centari and PPG's Delstar but they are pricey for what they are, in my opinion, plus Delstar may not be available at all in your area. It hasn't been available in mine for years now. There was a recent post by R Wyler just down a bit on this page who used PPG's ALK200, an acrylic modified alkyd enamel in PPG's industrial line and was happy with the result. That would be another solution short of the tractor store and dealer paints. Of course your paint selection will also depend on the ability to get it mixed at a local supplier in the proper color that you are looking for. Rod
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Rod