If you include the outer cover which should be held on by screws only, there are four- Outer cover, outer rocker, intermediate rocker and inner rocker. Work your way around the edge of the cover with a thin bladed scraper (Use non metallic if you are afraid of scratching). Perhaps the PO had it welded on for some reason? The three layers underneath are spot welded together and edge welded to the chassis at various points to form an attempted rigid web. Unfortunately, this captured area serves as a greenhouse for all the relatively unprotected metal to corrode from the inside out. Points of entry for water are the rear window from rain and washing, sunroof drain if you have one, and wheel splash from front and rear if the entry points at the ends of the rockers have been compromised enough. The outer rocker(not cover) has drain passages in the bottom which need to be open so that water can drain. Once these get clogged with rust chips, the rate of corrosion grows geometrically.
Extraordinary measures need to be taken to protect these areas from corrosion. You can get quotes from this board like- Don't wash/spray with a hose and don't drive in the rain. Do a search here to learn how many have shored up defenses against the tin worm in this area.
Sorry for digressing a little, but this one hits close to home. The battle rages on.