Rustoleum provides a nice anti-rust coating for surface rusted materials, however it doesn't like to be beating on, i.e. chipped at by road debris. Sand, gravel, road salt, etc will wear through the rustoleum.

What I would do is find yourself an ace hardware or equivilant, and purchase a bottle of "Ospho" (www.ospho.com). It has the consistancy of water, and you brush it on heavy out of a metal can of some kind, collecting what drips off into that very same can. One bottle was enough Ospho to do my crewcab's entire frame, inside and out, twice. And it's not very expensive either. I forget the bottle size, maybe it was a quart, but it was $10-$11 dollars. I also did my gas tanks, the bed floor underneath, the cab underneath, and various other things that had a ton of surface rust.

Ospho converts iron oxide (rust) to iron phosphate, an abrasion resistant primer that you can paint right over, with rustoleum or any other industrial or rattle-can paint. Ospho turns your frame into a pickled color, which is kinda funny.

Before:


After:



Before:


After:



In fact, in the bottom left corner of the last picture, you can see the bottle of ospho sitting on the ground. It's the clear plastic bottle with the green liquid inside.

When you use this substance, wear goggles as you will have some splatter with the paint brush. You have to brush hard and dab a lot to make sure you work it into the surface rust. Also, wear gloves, this stuff doesn't feel too good on your skin. I found out the hard way not to wear latex gloves - they dissolve slowly, and bond to your skin. Instead, buy a small box of nitrile gloves. You can leave the gloves in a can of ospho and they will be just fine the next day, so your skin is protected.

It's cheaper than POR15 by far, and it goes a long way. Be liberal and generous with your brushing... and again, let it drip off into the can. It takes a few hours to dry enough you can apply a second coat, and let that dry overnight before you prime or paint over it.