I would tig weld your pannels on they have less heat distortion then a torch and you can hammer weld. which means weld a 1/4 inch then hold the dolly on the back side of the weld and hammer on the fornt to strech the shrunken weld. if you can not get to the back of the weld you can use a nail gun to put a nail on the weld and use a slide hammer or a rivit gun with a cone shap puller on it to bring the shrunken weld back up. then you can fille off the weld and posible metal finish your pannel.If you are going to use lead or filler I would put it on 24 or 36 grit grinded bare metal the put the yellow rust preventer primer on It stikes to bare metal the best then put your filler primer on for the monior imperfections