Grind it out as best as you can. Your going to grind right through the wheel house don't worry. After its ground out as best as you can get it, Spray it with ospho. Wait 24 hrs and spray it again. Grind it back down and around it, inside and out atleast 3 inches outside the rust. Buy fiberglass matt, resin, and activator. Cut a template out of cardboard and shape your quarter and wheelhouse with this. Tape it in place. Cut your fiberglass matt into pieces of the right size. 4-layers should to it. Put your pieces on a piece of card board and spread the resin over them with a paint brush 1 at a time. After you have put on your fiberglass matt and it has dried grind it into shape. Repeat for the out side of the quarter. That is a tough repair for a rookie.

Your going to have to r&i the rear cover, tire, and the wheel liner. Your going to have to get up in the wheel house to do it right.