I have a 2006 SC430 with only 48K miles garage kept. About 4 years ago some minor surface rust started to bubble behind the paint just below the gas cap. I had it repaired at a local body shop and had no problems for about 4 years. I came back about 4 months ago. I had it repaired again and it is already coming back. The body shop said it is coming from the inner metal layer that he cannot get to. They are telling me new quarter panel. Anyone have any similar problem and do you believe they are correct?

 

Any time there are layers of steel welded and folded together that creates a moisture and rust trap. The only way to repair rust correctly is cut out the offending metal and replace it, how this is done depends on the location, the method used etc. there is no one size fits all answer.

Would need to see pictures to know exactly what needs to be done.