In the UK there are 2 products I would use for this issue. Not sure if there is availability or an equivalent where you are.
The 1st is Jenolite that is a rust cure that chemically stabilizes rust. You paint it on and let it get on with it.
https://www.jenolite.net/
The 2nd is Hammerite paint that is very heavy duty and can be put directly onto bare metal without needing a primer. It's as durable as epoxy paint without being anywhere near as brittle.
https://www.hammerite.co.uk/
I used a lot of Hammerite on a Beta 300RR that had paint that was as durable as donkey snot. The Hammerite colour was a very good match for the original Beta red.