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My bed has some serious rust/rot. I guess you could say I got lucky and was hit last fall by a Tanker Truck which took the side of my bed out. I took the $ for the repairs and bought a new 08 take off bed instead of getting just the one side repaired.
I actually just got done with a similar job where I used por-15 to protect my crossmembers. The stuff seems to work very well. I plan this weekend on taking my old bed off and working on the frame.
I've been thinking about this type of thing for my truck. The thin metal of these beds is just ridiculous.
I'd run a search on this site for rust treatment tips and discussions but the short and sweet is that most treatment liquids prefer some scrubbed down rust instead of bare metal.
The grinder is going to be too much of a fight. I'd lean towards a wire brush attachment and just scrub away the flakes and bubbles. Coat it down with the treatment and just follow the directions.
Simple as one, two, three. Three being the number of weeks this project will invariably take due to surprise discoveries.
I used POR15 for the first time this week on a rusty truck crane. That paint really impressed me. brush it on thick, and it dries smooth as glass.
Like Ray said use Ospho first.
POR15 has their own version called "Metal Ready"...good stuff