Originally Posted by MadMontie1
don't waste any real money trying to get that fixed. A few hundred maybe if you want, but not $1k.
The shop told you the right thing. The rust is forming at the seam, and likely coming from inside the welds to the outside. You can sand it down and repaint it, but the rust has "infected" the panel and will come back eventually.
Good advice. There is a reason rust like this is called cancer by those in the biz. Either live with it, or be prepared to move on to something else. If it were me I would choose the later.