+1 ^^
When welds "sit up" it means they are not penetrating, the metal is just building up on top of the work area. I've learned to turn up the heat, stitch the welds and use lots of compressed air to keep the area cool for the sheet metal on our cars. It's easy to get blow through so it's easy to fall into the trap of running low amps and lots of stick out.
take a look at this site:
Miller - MIG Welding Tips - Intro