Originally Posted by streetbruiser
If there is a lot of scaling rust probably best to sandblast, but I have used eastwood's rust converter and then followed up with rust encapsulater with lasting results. It's a about 100 bucks a gallon but you would have it for whenever. Clean it up the best you can with whatever you can get in there with first (wire brush and compressed air might be enough). Apply it thick with a brush, recoat after it dries and then wait a full 48 before applying rust encapsulator from a rattle can. You can use your primer over that too according to Eastwood.
As streetbruiser said the Eastwood rust products are good.... I used them on my Coupe after I blasted and then used a urethane product that dries rock hard. Get it as clean as you can, apply the Eastwood stuff, then seal it all up with a bedliner type product like Shake and Shoot.
Make sure you mask off properly though or you will have a lot of work to do to get it off of over-spray areas. Take a look at my Journal to see some pics.