Hmmmm, three words that you can only ever use two of at anyone time.
Want it cheap
and quick =No good
Want it good and cheap
Want it quick and Good =No cheap
Anyway, there is no easy solution to this. Unless you know what is on there as a barrier now, You can't do a patch job. You need to strip the coating off and start a fresh. Sand blasting to a bare metal is your best approach and then use an Epoxy
undercoat as a barrier. You are best to follow a recomendation of a recognised brand name.
If you think an epoxy
maybe on there now and the good area's look very sound, then spot grinding with a "Flap" type disc is best. But you need to ensure all rust is gone, or has been treated before you apply the barrier coat.
Do NOT dry sand any anti-foul paint
. The dust is highly toxic.
Hope this helps