I tried many over the years. I have one 4 runner with POR-15. Eastwoods internal Frame coating was a major disappointment. I coated that arch with that and went back in a year later and it was rusting. POR-15 and KB-Seal seem to seal off the steel. Just make sure you don't get any on your skin and where a respirator because those fumes are nasty. I just mostly use Gloss Black rust-oleum as my paint. Really wash out that inside frame and get all the dirt and debris you can out. Then just Fluid Film it the inside of the frame really heavily. When the outside is clean, you can either Paint with POR-15, KB Seal, Rust-Oleum the outside then coat it with Fluid Film. If you can even silicon the frames holes shut after its all cleaned up it will help keep the salt, dirt and water out. Making your Frame last a lot longer without repair..

I choose to plate the inside so that I can always change a Body Mount Etc if i need to and will have steel to weld to instead of plating the outside like a lot of people do. All your work is inside and you can sand it down smooth on the exterior which as you can see looks more OEM then just plating the exterior frame holes. You be surprised how thin some of the areas in the frame really are when you get some holes starting. That's why I switched do insides plates and replacing the bottom method.