Re: 2000 GTP...Retros, suspension goodies, 3.6, ...KR.
I have no experience with POR15 and among the rodder crowd, reviews are mixed. I'm sure its a solid product used in it's proper context and by the book.
What I do know is Eastwood. Like POR15, not available everywhere but all day online.
The project I left off this week would be a good example of what to expect but I kinda quit taking pictures of stuff, as you know.
In a nutshell... if I were you, and ready to lay in products for the job...
Proper abrasives and tools to run them (ask), compressed air and or shop vac plus solvent for cleaning metal, quality one step rust converter and brush, rust encapsulator or POR15, quality glossy rattle can engine paint any color, internal panel coating, patch metal, and possibly panel bonding adhesive.
Those last two only if you're cutting some out. As with anything, cleanliness is key. When the rocker claddings are off and you have flushed ALL debris inside the rocker away, then you scrape away anything loose that you can reach by hook or crook and flush it all out again. Air, water, whatever then blow or vac. Then sand off all you can with stripping discs, etc. Clean again. Apply converter. Dry, sand again.
You ARE taking plenty of GPF pics as you do all this, of course. Treat again if still brown, per directions. Apply POR or encapsulator per directions. Dry, scuff-sand, and rattle can that. Thick, many wet coats all dried good. Let all that air out then hose liberally with internal panel coating. Thats what Eastwood calls it, 3M says Rust Fighter. Thats how you treat rust you can't cut away, with a chance of long life. A wise man might plan to have that strut off when doing the tower deed.
I can link you to old rust posts of mine that could familiarize you more with the process if you like, with plenty pics. Already went into a TMI response though, ha. I believe in thorough explanations, I guess its the Dr. Bellows in me.