[masraum]

I'm pretty sure that step 1 is to remove the topper and check the extent of damage. I would assume that you'd then either need to grind down to clean metal or maybe that stuff that converts rust to a black durable finish is good enough, then paint.

I'm guessing that grit gets between the topper and bed and grinds away the paint until moisture can do it's thing. So I'd think that you'd need something to protect the top of the bed. I wonder if you could get a cap on the top of the bed out of a thin coat of spray on bed liner as a protectant for once it's repaired.

I'm sure some others will have better thoughts about how to fix the current damage.