The best thing to do to fill welds is : First--- Use an oxy-acetylene torch and a metal hanger and weld over existing welds. Second--- Get out a body hammer and a dolly. Third--- Now heat-up the weld and hammer weld a one and a half inch spot flattening out the weld (this also flattens out the weld on the inside) move at this rate till all the welds have been covered and flattened. If you did this method correctly, you will not need any filler at all and the pieces welded together are very strong and will look like a FACTORY connection. If you still want to fill the welds without using this process, I recommend a filler that is waterproof and will never lift like Bondo tends to do. Try using ALL METAL or METAL-TO-METAL these are aluminum based fillers and are guaranteed not to lift or allow any moisture to penetrate through to the metal. Good luck