My day job is coatings.
A good sound coating on anything is a great base for new coatings. If you can't scrape it off easily, then scuff/sand and apply primer. The kind of primer is an important step. If there is any bare steel, then the only primer to use is an epoxy primer. Not "epoxy fortified" or some other sales jargon... simple 2 part epoxy. I don't care what brand you pick, just as long as it's a true 2 part epoxy.
Epoxy on bare steel is the ONLY thing to put on bare steel. The reason is that if epoxy gets scratched or chipped, the spot damaged will rust BUT the rust will NOT creep out from the damaged area. The epoxy will prevent the spread of rust. This is why an epoxy on bare steel is so very important. No other products even come close the ability of rust prevention like any 2 part epoxy. Period. End of story.
If you want to paint the epoxy you can. Generally, epoxies get harder over time. They need to be scuffed or lightly sanded to open them up to accept a paint. A good 2 part urethane paint is a great product to put over an epoxy as it will retain it's shine, be chemical resitant and abrasion resistant. This is why an "epoxy/urethane" system is so very popular industrial and commercially speaking.
Getting back to the the frame with some "tightly adhered coatings" you can scuff and prime with an alkyd paint primer but the key here is a "good primer". It will help the top coat last 5 times longer than top coat without it.
There is no substitute for 2 part products that require 2 coats of product. Therefore epoxy/urethane. If you decide to go with 1 part products and no epoxy, go ahead. Just know you are NOT getting the BEST possible coating to prevent corrosion and rust.
