Had a customer drop off an exhaust manifold for a boat that has some cracks in it. Figured I'd do like I normally do and tig it with silicon bronze. Soon as I form a puddle it erupts and makes a nasty mess. Would mig'ing or smaw with nickel be better? Do I need to drill holes at the front and end of the crack?

I use high nickle from Washington Alloy. I drill the holes at each end and grind out the crack, not all the way through. NO! preheat. Weld as cold as possible while getting good wet edges 2" max at one time single width pass DCEN. That is the width of the filler metal including flux coating. I immediately peen the weld with my slag hammer while it is still hot. I have had excellent success with this method for thirty years.
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Dennis