Dark Green is a pretty color for a roof (pretty hot!) You lookin' for solar heat? I would recommend the aluminium (silver) colored stuff. As far as the rust is concerned... There are products for treating rust, chemically passivating it, not a remover like "Naval Jelly" or its copies(all based on phosphoric acid). There are primers that will go on over "LIGHT" rust not flakey scaley chunky stuff. I have used some of these that are water based and some that seem to be in a kind of lacquer base (???) The water based is easy cleanup before it dries. Went on milky and dried, where it contacted rust, nearly black. Painted over it with oil based paint and various catalyzed paints from vinyls to epoxies with excellent results. This was in the bilge of an ocean going sailboat (think corrosive salt water environment) on the threads and nuts of the keel bolts (think - if the keel bolts fail yoiu might be dead soon) I was pleased with that stuff. I think it was a Rustoleum brand product, a tad pricy but I didn't request a palet of 5 gal pails, price. I got it in a tall thin plastic pint container.
Now the other primer. Got it at Home Depot (AKA the BOB - BIG ORANGE BOX) it was a spray can. Went on like a clear plastic spray (but thinner). Started drying clear except where it contacted rust and then turned dark purple. I then coated it with spray enamel (Krylon, Rustoleum, whatever.) It too worked good but you need a volume price and putup to be ecconomical.
And finally (whew!): Rustoleum fish oil based damp metal primer (dark red). Smells fishy, is thick and coats well. Dries kinda slow A N D lotsa spray paints aren't compatible with it. Trying to topcoat it is not a joy with say Krylon spray enamel. This would not be my first choice.
Have you enough rust to require sanding? If yes, consider DIY sand blasting, lightly and only where needed but paint the cleaned metal as it will almost rust again as you watch if there is much humidity.
You could go with a phosphoric acid wash (active ingredient of naval jelly). Spray it on with a pump up sprayer for larger areas, too big of a job for a plastic spray bottle. Rinse it off. Let it dry, if drying conditions are good. If not try blowing it with output of shop vac or shop air. Then use primer as above.
Hope this wasn't too confusing... and that it helped. There is no easy way to do the job but there are plenty of hard ways and wrong ways.
Patrick
