In the picture that is posted it looks like you are using a metal grinding disk to take off the paint in the trucks bed. I would suggest that you use a grinding wheel that looks like a green SOS pad; these are sold under the names 3m Roloc Disk, or Clean and Strip Disk (google it). The reason for this is that the grinding wheel you are using is removing too much metal (causing burn through) when all you want to do is strip the paint and remove contaminants. The roloc disk will strip the paint, lightly abrade the metal, and not leave behind any contaminants.

In your circumstance using a buzz box and some 0.035ö flux core wire is not ideal for welding sheet metal. It burns too hot. Like others said get the GMAW setup and thinner wire but if you cannot afford investing into the setup right now you could get away with doing some non cosmetic sheet metal work in your trucks bed with what you have right now.

Use these pointers that have been passed to me and I have figured out while welding hundreds of patch panels in my toy with the same flux core setup that you have:

The most important thing to do is prep the metal with proper fit up and cleaning. After you cut out the rust and prep the area with the grinding wheel clean it really well with a cloth and acetone. After you remove the corrosion and mill scale from the edges of the patch piece with a wire brush clean this piece by wiping it with acetone as well, even oil from your hands will contaminate the weld. Make sure the paint and all contaminants are removed at least 1ö around the weld.
I find that it is easiest to cut the sheet metal so that it covers the hole like a patch so you are lap welding it just like you are attempting in the pic above. Cutting the sheet metal so that it fills the hole (butt joint) might look better but it takes waaay longer and is much harder to weld into place.
As others have said ôTACK WELDö the sheet metal only, disperse the heat by evenly spacing tacks out and moving around the panel to be welded until you fill in the entire surface of the joint to be welded. Get you welder dialed in by turning the heat down and wire speed and practicing a lap joint on scraps. Get a good ground!!

Fit the patch to the truck bed so that the gap between the base metal and the metal to be welded is less than the thickness of the welding filler wire. Get comfortable aim the gun and pull the shade (or nod), make your tack and move on to another spot to let the tack cool. Hammer the sheet metal to shape as you go with a ball peen hammer or body type hammer. Push the metal together with the hammer, make another tack and move on. Repeat hundreds of times.

If you still burn through a couple of times (and you will) grind the weld down and weld little tacks on the edge of the hole until it is filled. Make another patch if the burn through hole is big and thin. Try preventing burn through by aiming the gun more at the base metal (the truck bed). The reason for this is that the edge of the sheet metal heats up quicker than the base metal (think of lighting some cardboard on fire, the edges and corners will burn quicker than the middle of the sheet).

Also remember that flux core wire will deposit flux on top of the weld, for best results remove the flux from the tack weld before you weld beside it. Use a chipping hammer, or your trusty lil wire brush!

Well give this a try! I am sure it will help; it just takes a lot of time and patience! If you are fed up with this route and want to consider using pop rivets DONÆT. Use sheet metal screws they are quicker and work better.

Cheers and happy welding Brah