your link isn't working.
Try turning the numerical setting down on your welder.. It should have a guide for settings based on thickness of the metal.

Other than that, welding on sheet metal is a real PITA. Most of the body work guys I've seen start with tacks. IE - a spot here, a spot there. They fill later with a TIG. Filling heats up the metal and tends to warp it... I'm suggesting that the best way to do it with a budget welder setup is going to be to cut out the offending metal, fill with new patches as appropriate (consider counter sinking instead of what you're trying to do), spot weld, - and then fill with a fiberglass resin based filler (not bondo). Dynaglass comes to mind.

My brother built a gas tank with a MIG, it's possible to do but takes some practice.

A flux core welder makes pretty ugly welds anyway! You'll be amazed if you ever use a decent mig at how much cleaner and easier it is.