In doing some minor rust repairs on my driver's side long, I decided to do a little exploritory surgery. I cut a small part of the outer layer of sheet metal out on the the inner wheelhouse (at least this is what Restoration Design calls it, I think) and found some mild surface rust. It looks a LOT worse in the photo then it actually is as what is shown in the picture really is all just surface rust. (sorry for the bad photo- camera phone) I'll be welding
on a replacement 'outer layer' of metal just as soon as I can fabricate the piece.
There was NO sign of this on the outside of the wheelhouse- I only cut into it because I had seen some rust in between layers of sheet metal further down towards the front end of the car on the long on that same side.
It might not be a bad idea for others to cut a small square of the first layer of sheet metal out on their cars in the same spot just to make sure all is what it appears. A dremel tool makes it real easy to just take the outer later off- it allows you to cut really slowly and NOT go into the 2nd layer of sheet metal.
Rust is fun!