That is some serious looking rust. only way to get rid of it is to cut out all the rusted metal and replace with new and get it sealed real good. need a welder to do the job right, don`t want some one doing a bubble gum weld job. you need good weld penetration for the structural integridy of the body. Buy some new Rockers and do it right.
I haven`t tried it yet, but a guy told me to use Acetone in a spray bottle to soften the under coat sealant. then it can be removed easier. also MEK will work, but more leathel to breathe.
I found an almost perfect set of Ivory fenders in PNP for my 240. they had a 50% sale in a few days, so I removed all the bolts on both fenders. day of the sale figured I would just pop the fenders off and be gone. But NOOOO, they were glued on.

I used my razor blade knife to cut the stuff, but never did remove them.
I`ll try the Acetone next time, wonder what it would do to the plastic spray bottle.
Charlie