I don't use backer bars right now probably because I don't have any, lol!
Mid panel body seams take me a long time to do because of heat concerns. I am going to get some if that heat sink clay that you can flank your weld with.
Well with every addict in the universe putting the gleep on anything vaguely resembling copper, HEAVY copper's gotten harder to find. Will get you lined out with some,....I use a LOT of carbon graphite on heavy wear metal for backers and reestablishing Plow bolt holes and such, but it's a BAD mistake to go anywhere near the light gauge stuff with it, due to carbon embrittlement.
The problem I have with that "Helping Hands" clay is that it gives up steam and disrupts my gas shield,.....I do have some clear gel that you can paint on, or dip and let dry,....don't ask me how the stuff works, but you can hold one end of a light strip of sheet metal bare hands,...and heat the other end HOT with impunity / no discomfort. My welding supplier's closed today, so I can't call him for the name of the stuff
