[dsraven]

a few tips that work for me. right or wrong, dunno. not a pro by any means. take with a grain of salt I guess.
first, the metal needs to be clean on both the front and back sides of the weld before you start or you will draw contaminants into the weld

second, always use the mig wire cutters on the wire before you strike a new weld. the sharp tip on the wire will chisel the surface and start a clean weld right away so less time "waiting" for the bead to "clean up" and start welding nice. by that time you may have burned through.

third, do short bursts/tacks on suspect thin areas. stick a small "ball" of metal on the edge with a tack. that makes the edge "thicker" and it will absorb a bit more heat from future attempts to close the gap. use a hammer and dolly (hammer on dolly) right away to help flatten the ball after and this will bring the 2 sides closer together as well as assist with the shrinking issue when welding sheet metal. a weld "ball" on each side of the weld can be connected with another tack after the area has cooled off. again, hammer and dolly (on dolly). allow stuff to cool between welds.

fourth, move around the patch to ensure shrinkage/distortion isn't creeping in on you so your patch doesn't fit nicely anymore. I usually have success with tacking 4 sides, then put tacks 1/2 way between those points and so on all the way around the patch. you can use compressed air to help cool the entire area right down again before welding more. use the air after the hammer on dolly part is done. hold the blow gun further away from the surface than your brain tells you to because that will help cool the entire area instead of just the weld area.the weld area wants to shrink when it cools so weld, hammer on dolly while it's hot to stretch it, let it cool to the same temp as the rest of the panel/project, adjust with hammer and dolly as required to get the shape needed and keep gaps etc, go again with more tacks

fifth, check the thickness of the metal around the patch compared to the normal metal thickness of the surrounding metal. if it's a bunch thinner then you may need to go bigger with the patch to get out further where the metal is thicker

sixth, if using a slightly thicker patch material than original steel in the area of the patch, possibly because the patch panel doesn't come quite as big as you thought, start the tack on the new steel and wobble it over to the old steel quickly then stop before the heat causes burn through the metal

seventh, a copper or brass backer with somewhat of the same shape as the weld area will help by absorbing heat plus weld doesn't stick to it. it also helps keep the puddle from falling through the weld area, which makes a burn hole, which then makes the gap between the parts bigger, which takes more welding to close, which means more heat to the area and/or more time to finish the weld, which usually means more flap disc time, which means more heat build up, which means more distortion, which....you get it. you can use a flattened out piece of copper pipe. I have a few of different sizes and shapes from different diameters of pipe and have found them handy for this. a small handle can easily be put on them to stop heat transfer to your welding glove. a well placed magnet or vice grip can also be handy to keep them in place. magnets will cause disruption of the pig weld though.

eighth, have you played with wire speed and voltage settings (I assume you are mig welding) to see if a lower voltage or more/less wire speed would help?

ninth, sometimes what has worked for me is to start a tack, with a bit more wire speed, but make it short, time wise, and leave the wire in the weld puddle. let it cool off for a sec and do a second tack right away. there is no waiting for the weld bead to strike and cleanup because the wire is already embedded in the metal. this will allow you to get a bigger "ball" of weld metal on the edge of a thin metal patch. this also works when you have good fitting well spaced parts and don't have to worry about burn through so much, you just want to lay down a longer bead without heating up the area and then possibly burning through. you can do a short burst, leave the wire in the bead, wait for the glow to fade through the welding helmet screen, then do another short burst and pull off. hammer on dolly, allow to cool, do another spot somewhere else on the patch.

tenth, when you have an area with some of those weld "balls" built up on the edges you can connect them with a short burst in a circular pattern and watch them fill in the middle themselves. a copper backer helps here. then hammer on dolly right away, cool, metal finish, check for pinholes with a bright light on the back side in a dim area, epoxy prime, fill as required, etc. you can also check for pinholes using solvent or brake clean sprayed on the back side of the patch. it usually finds it's way through the small spaces especially if "helped" with some compressed air (use a solvent that doesn't evaporate as quickly or use lots). if you get a wet spot on the front side then you have a pinhole. a helper is a good thing to have for this job. remember the solvent is flammable so be careful with it when welding/grinding etc. it's a good idea to have a fire extinguisher around closeby anyways, whenever welding/grinding etc

that's it, I'm outta fingers, you got 10. lol