Before applying the paint one key prep step is to get off all oil/wax/fluid film/etc. FF can be very difficult to remove entirely.
A painter friend explained that he uses oven cleaner for this. It is made from lye, which eats organic compounds (especially oils) and doesn't harm paint or steel (if left on for a reasonable time). Wear good protection on you, and mask off any areas you don't want etched (i.e. hold a piece of cardboard between the spray can and objects you don't want exposed to the spray, as you move along), then clean the de-rusted rails with it to be sure to get a strong adhesion between the paint and the clean steel.
I just took my OEM running boards off to hammer-on-bench-vise the brackets back into a viable condition. I cleaned the running boards up and applied a coat of that cheap Rustoleum bed liner. I couldn't help but notice all of the rust scale on the major frame components of course. I plan on keeping my FJ for a very, very long time. So addressing the frame rust needs to be on the short list.
Following this (and 1,000 other) frame rust treatment threads. And buying a pneumatic needler!!
I have been using the mini scaler from Harbor Freight, it has done a good job but the mini don't have replacement needles so get the bigger one. I used Rust-olem Reform and Seal on the wifes 2 years ago and held up in most spots. Going to touch hers up then Fluid Film the whole thing (that don't sound right) lol.
My frame maintenance consist of hose down, degreasing, wire wheel and hand brush for those hard to reach areas, Ospho rust neutralizer, Eastwood primer,Eastwood black chassis paint and Amsoil heavy duty metal protector. I find that fluid film washes away to easily.That Amsoil product is a cosomoline wax which I out found during my Navy stint is quite a preservative . Since switching to that heavy duty metal protector I'm finding I'm doing less touch up.Take your time preparation is 98% of the battle.
I used a piece of garden hose clamped to the A/C drain to move the condensate from dripping on the frame. Eastwood also makes an internal frame protector spray which you stick into the frame holes you will need to buy the pancake attachment which sprays 360 inside the frame highly recommended.I would use that Ospho product first before the frame treatment.
My summer project is to remove the rear bumper valence so I can see the exterior of that rear bumper. There is a Facebook page dedicated to rusty FJ's ,that is one group I do not want to join. I need a tetanus (sp?) shot after looking at those pictures.
This will be my 8th year of frame maintenance that I've learned through trail and error. The above is what works for me living in the North East rust belt. My .02 Rust Never sleeps Neil Young.
Full disclosure I have no financial position in Eastwood nor Amsoil I just like their products.