Cleaning lathe castings before paint

I'm sure this is a really stupid question. I am getting ready to paint the pedestal of my south bend 13. I wiped it down with solvent but the paper towels keep coming off black or grey. I stripped all the paint off down to bare metal a while ago with a wire wheel on a drill. would whatever keeps coming off harm the paint or bondo down the road?

My gut feeling is its not clean. I prefer to wash with Zep floor stripper (some prefer Purple Power) and HOT water. Soak it, scrub with a brush and then rinse with hot water and dry afterwards and you shouldn't have rust issues. For items this large it helps to have a garden hose that can be attached to a hot water tap. I use mine straight out of the tap at 120F. YMMV

The black stuff you are down to may be the filler used to smooth the casting preparatory to painting. I've always used Dow oven cleaner to de-grease big castings. Wash it off with water and you are good to go.

Iron tends to leave some grey on the rag from all the graphite in it. As long as there is no oil on it, the paint should stick fine. After degreasing mine, it got a final wipe-down with acetone before paint. The rag did turn dark.

Everyone has their preferred degreaser, but most of the water soluble ones are lye based. It all works about the same at the end of the day. You could probably soak these things in Drano and get the same results.

I have painted several machines over the years and like Mike said I had the best luck using super hot water and an industrial soap of your choice. Go to Rag-Stock and buy some cotton rags, dump the paper towels. After I wash the crud off, I sand the bad spots or scrape off the lousy paint, wipe with a good Fast to Dissolve Lacquer Thinner wearing rubber gloves and with a window open, then prime using a Sherman and Williams "auto body" primer as I found their primers for metal far superior then other brands and paint it. I used a good Alkyd paint. Use a brush or foam paint roller. I have heard a good tractor paint works good too. Rich

The casting is down to completely bare metal, no paint or filler or anything else to come off as far as I can tell. Wouldn't rinsing bare metal with water cause some flash rust?

Depends. If its after a bath in caustic solution, something about the PH actually helps keep it from rusting. If its hot water, it dries fast too so it doesn't tend to rust since water doesn't lay. Mine didn't have any problems with it at least, and my well water at the time was nothing short of horrible. I didn't leave the raw iron laying around very long before painting though, basically long enough to blow it dry with air and lay it in the sun for a short time to fully dry before painting.

Alright, thanks for the replies. I'll rinse a smaller bare part in water first and see if it rusts at all. If it doesn't Ill give it a shot on the big casting. It sure is a lot of trouble getting it over the step out of the the basement though.

Anybody use OSPHO aka phosphoric acid to clean and prep iron base castings? I've used it for rust removal and paint prep on all sorts of steel parts, from boat engines to railroad wheels, but have no idea how it would react to iron.

BTW. OSPHO is also an excellent cleaner. It seems to attack many forms of crud and oil based residue. Wipe on, wait five minutes, and wipe off with a cotton rag or towel. Strips fiberglass boat bilge grime amazingly well, with no damage to gel coat or enamel paint. So it probably would do a nice job cleaning any residue left on your SB base, and Definitely will prep the surface for painting, except I've never used it with cast iron...

Today I took the door casting and scrubbed it well with a plastic bristle brush under hot water twice. Both times a little metallic looking dust came off. Afterward I dried it and poured a little mineral spirits on it and scrubbed with the same brush. Same black/gray wiped off afterward. Here's some pictures of the door.

I haven't wire brushed the rim of the door so don't pay any attention to that.The little bit of black isn't any filler. I think its how the light catches on a smoother part of the casting. I don't think cleaning the castings with purple power and hot water would do me any good since mineral spirits and scrubbing in hot water certainly didn't. I didn't touch the bigger casting with any water at all and I noticed if I scrubbed it with mineral spirits it seemed to raise up some more black stuff when wiped off. Whatever is coming off when I wipe it isn't noticeable on the surface of the casting though. Its kinda like a light dust that wont ever completely wipe off. Does anyone have any other ideas about what this could be? since its not a loose dust on the surface I think I might just go ahead and wipe it down with acetone or other solvent and paint if nobody has any more ideas. Thanks for your help!