As kentuckid mentioned, this product is definitely worth a look for cleaning and de-rusting tanks. IT WILL NOT DAMAGE PAINTED SURFACES, NOR WILL IT DAMAGE YOU.
Yes, it's expensive, BUT CAN BE RE-USED SEVERAL TIMES FOR SEVERAL TANKS. I store mine in a 5-gallon gas can I bought for the purpose. IF you absolutely, positively do not want to re-paint your tank, $125 is pretty cheap, IMHO.
I have some working in a BMW tank right now, and will let folks know how it comes out. My previous treatments have been on an old Goldwing tank, which had no coating (other than RUST!) that I could detect. Goldwing tanks have "internal plumbing" that (generally) means you CANNOT use "coatings" successfully. Also used this SAME (used) batch on another tank (also "uncoated"), and it makes the internals shiny-new IF the only problem was rust.
I cannot (yet) address how successful this product would be at removing flaking original red coating, but will pass that along when my current "soak" is finished. I suspect that it will NOT remove old original coatings, nor would it remove POR-15 botched jobs, but that's just a guess. The techies @ Rusteco can tell all, or maybe their website can do.
Check out the website, and call them. Their "main" mission is de-rusting HUGE commercial equipment, such as bulldozers that have been buried in the South American rain forest for months and months. (Not kidding.) The workers using these big vats don't even have to wear protective equipment beyond what you'd wear to immerse your hands in water all day. . .NO fumes, whatsoever.
Once treated/de-rusted, this product provides an invisible layer of protection so it won't rust again. My old 'Wing's tank still looks great after two years, and this bike sits a lot, sometimes with only a half-tank of (Sta-bil treated) gas.
NO affiliation with these guys, beyond knowing that the product WORKS, as advertised. They'll also do it for you, out of L.A. Send them your tank, if you want.
Several months after coating the tank and having cleaned and reinstalled the fuel petcocks, I replaced them. I will eventually rebuild the originals, but having clean new petcocks is very nice. The new petcocks have a distinctive 'click' when rotated. Part of my reason for buying new was from running out of fuel on two occasions - because I reassembled the tubes backwards - and did not want a third incident (photo's should have been taken before disassembly). Since you have photo's that should not happen in your case.
Nice, huh? I finally broke down and sprang for new straight-through Karcomas (from Bing) for my toaster tank. I was going to swap them over to my 6-gal tank, but decided to just buy another set of new ones for it too.Several months after coating the tank and having cleaned and reinstalled the fuel petcocks, I replaced them. ... The new petcocks have a distinctive 'click' when rotated.
"It is what you discover after you know it all that counts." _ John Wooden
1973 R75/5 - original owner
2008 R 1200 GSA
1975 850 Norton Commando
If it ain"t broke... fix it till it is.