A few years ago I had a shop replace almost all the sheet metal on my car to include the rear quarters, but they did not replace the rear wheel wells, and I just found out that they have a lot of pin holes at the top of them.

I do not wish to remove the new painted quarters to get to them ... too much money, so I plan to cut out the bad areas, and weld in new sheet metal. Keep in mind I have never done this before .... What type of metal and thickness is best to cure this problem?

I know that 10 years or so down the road I may have to relook this problem again if I don't do it right the first time.
Thanks.

 

If the basic structure is still there but you have a bunch of pinholes, I would clean the wheel wells off as best as you can, mechanically remove as much of the looseish rust as you can (knotted wire wheel on an angle grinder will do this quickly), wipe the whole thing down with a phosphoric acid metal prep and then use fiberglass and por 15 to repair it. Done right that should end up pretty much a permanent repair, and can even be made to look good if not next to perfect.