Sand it with 80 grit to get the heavy stuff off, wipe the junk off the surface, then apply Picklex 20 several times with a brush. Picklex is a one-step metal prep that works really well. Not near the hassle of jells. Resto Rick turned me on to it. It's environmentally safe and non-toxic.

How many times you keep brushing it on depends on how much rust is left after sanding, but that stuff will eventually get all the rust off and the rust it doesn't get off, gets neutralized and is ready for primer.

Keep the surface wet with the stuff and keep brushing it on every few minutes. Do smaller areas at a time if you don't have time or patience to do it all at once. Just wipe it dry when you're done. Not as fast as media blasting, but a whole lot cheaper and it works.

I've used that stuff a lot and it works better than any other bottle-type chemical I've tried for surface rust. And there's no prep like POR-15's rust remover. It's an awesome one-step metal prep. It gets down into the pores of the metal and kills/neutralizes the rust, but if you keep applying it, it will eventually turn the rusty metal into a clean bare metal surface. Not as exactly clean as a media blasted surface, but the rust will be killed. You can hit it with a scotch brite to work in if you want.

I've had pieces setting in primer for several years after applying it and no problems.

It etches the metal so do not prime with a self etching primer, use an epoxy primer. Before you prime it, hit the surface with 80 grit again and wipe clean with degreaser just to be sure you have plenty of tooth in the metal for the primer.

It may seem expensive, but one of the small bottles goes a very long way. Easily enough to do your roof and you'll have some left over.

I've tried the Ace Harware version of it, called Must for Rust. It works okay, but takes several times longer than Picklex 20 and never does achieve the same results.

I did get a free bottle of Eastwood's Fast Etch. It appears somewhat similar to Picklex, but requires you to cover the surface in rags. That sucks. And it actually eats away at the integrity of the metal if kept on too long. Picklex doesn't do that.

I sound like a salesman, but I'm not. I've just had good experience with it.