POR 15 is the older formula. I used both and I perfer KBS seal over the POR15.
Internal frame
1.) Power wash and clean all the dirt and debris you possible can get out of the frame you get out.
2.) Being scaling will be taking place. you need penetrating oils etc on the steel. FF will cover steel but will not penetrate the scale. (FF is good for a Top coat on a rusted frame especially if scaling is occurring)
3.) Use penetrating oils or fluids to coat the inside of your frame. A million choices here. Examples: Used Transmission fluid, WD-40, PB Blaster. Anything that penetrates and will seal the oxygen off the steel. So rusting will slow down and stop.
4.) Topcoat with FF or anything similar that will creep around inside the frame. Cosomoline, Woolswax whatever works for you.
External frame and underside:
1.) Power wash and clean all the dirt and debris off.
2.) Descale everything you can. Wire wheel etc. Get all the scale off and loose rust.
3.) This phase I am still experimenting on. Use a good Rust inhibitor on the steel to seal it off. Do not just primer and paint. it will just rust through that in the rust belt. I don't care what kind of paint its not going to seal it good enough.
Right now I am experimenting with Corroseal as my rust inhibitor layer. As its a metal etcher and sealer as one. I will not know how it holds up for a year or 2.
Even KBS seal will rust through if the metal wasn't peeped properly. They all will.. That part of the key is getting the metal clean and prepped to seal properly which is extremely hard to do upside down and underneath a car..;-)
4.) The topcoat can be whatever works for you paint, FF film wool-wax. Comeliness.
Note: (FF will not stay in fender wheels or areas that see a lot of pressure washing style actions) So either touch up every year or find something that better for that area.

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