[paint guy]

Originally Posted by 69Customs

I use Por15 as directed when possible removing rust and applying to bare metal with the phosphate as directed. I never spray as that is not recommended. I used it in the inner truck lid when I had the lid off, Cleaning out as much inside without taking it apart. I then poured POR15 into it with it upside down and swished the trunk lid so it came out all small drain holes in corners. WHY? Cause I can't get into sand surface rust out and paint. I wanted all inside creases to fill with it as it is a moisture curing paint. The more moisture the harder it gets. I figure it will block any air or moisture from getting to any surface rust and seal it from going any farther. Regular paint breathes and can let moisture thru.
So I figure something to seal light surface rust is better than nothing. Plus POR15 you don't have to mix like epoxy enamel.
That's why I use it and have had good results.

Short of chemically removing the rust by dipping, this is probably as good a system for mitigating rust inside a trunk lid as any that is available to a home restorer. I happen to use a different method, using either an aerosol (rust-convert" type product or a combination rust-remover/phosphate conversion coat type product that I will let dwell in inside areas, then drain. In either case, I will then spray with an aerosol can "cold galvanizing" spray paint (over 90% zinc) or pour in epoxy primer in the same manner as detailed above. Coming from the factory, the inside of deck lids had a conversion (phosphate) coating on the inside, but no primer or paint (other than unintentional overspray). most of them also had some kind of sound deadening material which of course, would attract moisture.