i use kbs rust sealer, sticky stuff, i did my whole floor pan with it and inside the plenum/cowl, as far as i could reach, wear gloves tho, i had black stuff covering my hands for a few weeks, if your quick petrol gets it off, got it in my hair somehow too, had to cut it out lol, it ran out onto the cement on the floor, and trying to get it off it was ripping up part of the cement so it stayed , its the same/similar to POR15, ill do the insides of my doors and guards also,

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i also did the inside of my tailgate on the frame and skin when i reskinned and reinforced it, another thing i use is rust convertor, cant remember the brand, its good for a year or so to protect bare metal,