Can I paint over the factory primer?

I've enclosed a picture of my NOS engine block that still has the original primer on it. I'm checking to see if I can paint over the block and still keep my beautiful paint job. I'm planning on painting the tractor with PPG with some added hardner. I'm by no means a painter, just a newbie.

It sure looks like Glyptal, especially since it is on the inside also. Glyptal was (still is) a very common specialty paint/sealer for use on the interior of cast blocks, gear cases, etc. to seal and smooth out the as-cast surfaces. I believe it is an alkyd-based enamel. Since it is a NOS part, the paint would be fully cured and I doubt you would have a problem in coating over it. Do as Frank suggests though and use a little lacquer thinner on a rag and rub it in a small area. If it is Glyptal or some other enamel, the color shouldn't come off with a lacquer thinner rub.

If it does come off with such a rub, it would be best to remove it on the outside areas. If it doesn't, well, I'm not convinced that it absolutely has to be removed. You'll need to consider how you are going to remove it. The best way would be to sandblast, especially since it is a bare block. That may not be an option for you. The other ways are chemical stripping and wire brushing. I'd try to avoid the chemical stripping since I would be concerned about some stripper remaining in the pores of the casting. Wire brushing would be my choice after sandblasting, provided it came off relatively easily. If it didn't, and it appears sound, my tendency would be to simply clean with something like PPG DX330, scuff with a Scotch-Brite hand pad, apply a single coat of epoxy and proceed to topcoat. If it wasn't an OEM application, there could be rust underneath or it was otherwise poorly prepped and I would agree with the others and say remove it anyway. Either way, I wouldn't attempt to remove it from the interior of the block. I doubt you are going to topcoat the interior and you would be unlikely to improve on anything by removing it from there, especially if it is Glyptal.

If you do remove it on the outside, be sure to use a two-part epoxy primer in it's place. Unless you are planning on epoxy as a replacement, you might as well leave it alone, as long as it appears in good shape. Make sure you understand the exact time window for topcoating any epoxy product you might use. It can be anywhere from hours to days, depending on manufacturer and product.

Rod