Black Rust?
 

Decided to strip the paint off my old '76 Nishiki International and repaint it and tune it up so my son can ride it. I am seeing black spots on the bare steel that are hard to sand or brush off. Is this black rust? Do I need to completely remove it before priming? I was planning to spray the frame with a can of self-etching primer before the color coat.
https://cimg2.ibsrv.net/gimg/bikefor...72bc8d012e.jpg
https://cimg3.ibsrv.net/gimg/bikefor...a7fccdb1a4.jpg

That very clearly looks like rust. Pinholes in the paint allow oxygen in, and rust keeps pushing to the path of least resistance. A self-etching primer should do the trick, but starting with a smooth frame pays dividends in the end finish. Any roughness should be sanded out level with the surrounding steel, and any pits filled with body epoxy.

For peace of mind, you can use a phosphoric acid rust converter in the frame tubes to arrest rusting there, and convert the rust to an inert and somewhat protective iron phosphate.

I can't tell what tube the top photo is. I'd guess the bottom of the down tube or a chainstay though.

Yes, bottom of down tube. Also see it on the chainstays and seatstays. Will take your advice and do some more wire brushing and sanding tomorrow. And pick up some rust converter for the insides. Thanks a lot for the help!

Want to find out? Dab a little Naval Jelly on it for 10 minutes, then remove it. It's paint-safe if not left on for more than an hour or two. Remove it. Is it gone? If so, it's probably rust.

:thumb:
Thought so. Any stone chips flying off of your wheel can break the paint film enough for oxygen to get to the steel. The bottom of the down tube and insides and bottom of the chainstays see the worst of it.

Thanks! I almost got some naval jelly last night but ended up getting CLR since I've got to work on lime deposits in our bathrooms also. If CLR and additional sanding doesn't work, I'll get some naval jelly. Maybe overdoing it for a non-enthusiast, non-collectible bike but it still has some sentimental value.

Naval Jelly is a no-sand solution. It just dives in and eats up FeO. Just need to rinse it properly after. It's really a no-brainer solution. Just RTFM on the label and you'll be good to go.

But I've never worked w/CLR so maybe it's easier? Dunno.

I’ve used CLR on chrome. It’s fine.
just brush it off after about 15 minutes. Reapply if needed.

Most CLR brands contain a "blend of acid" that includes phosphoric and/or muriatic. Should be fine and effective if you follow the advice above.

CLR has been reformulated to avoid dumping P ions into the water. CLR ingredients are a blend of organic acids that are less likely to harm the item you're trying to derust/descale: Lactic acid, citric acid, and gluconic acid.

Thanks everyone. CLR plus additional wire brushing and light sanding seem to have taken care of the black stains. Hope to start srpaying primer tomorrow, if the breezes die down. If all goes well, I may post photos with the bike is back together in a month or so.