Once you're satisfied with the fit, permanently secure the new doorskin to the frame by welding the flange every 4 to 6 inches. Check the door's alignment after every weld to make sure it didn't shift. If you have problems with burning the material even though your welder is set as low as it will go, try increasing your wire-feed speed and weld in short bursts until your tack is a satisfactory size. Keep in mind that the cleaner and flatter the weld is, the less cleanup you will have to do later. Once the welding is complete, use an angle grinder to finish the welds along with a thin bead of seam sealer to keep the rain out. You may now sand and prep for paint as needed.