Solving this issue involves removing all the rust. The shop didn't or the problem wouldn't have come back. Some peeps use sand, others hardware nuts. I prefer to use steel BBs like the kind you shoot from a BB gun. Get it thoroughly cleaned and rinsed and then use something like Metal Rescue to remove the rust that remains. I'd steer clear of products that coat the tank as they can cause more problems that are near impossible to fix. Good luck.
2013 V Star 1300 Deluxe
OEM Tall Sissy Bar
Memphis Shades Fork Deflectors
Bridgestone Exedra Max Belted Tires
Baron's RSTD 7308 bar