it depends on how you welded it, if you mig-stitched, hot is the only way you'll get it to move. If you tig welded it, you can do it hot or cold. The biggest thing to remember is, when you're whacking it, that how much metal moves depends on its alloy. Mig wire cools to a pretty hard material and can only be moved with a BFH and questions about its heritage, whereas if you tig with silicon bronze, that will move with a stern look and a brush with a feather.