I don't have good news for you, rust is like an iceberg, what you see is just the top of it.
IF you want to try to slow it as much as possible, you're going to have to dig/grind/wirewheel as much of it out as possible. Then use a rust reformer like naval jelly, ospho, etc. Then paint it with something like eastman paint. Then fill it to make it the right shape again if necessary, then paint it back to the matching color. Even then you're just buying time. I'd assume the other side is about the same condition too.