Yacht welding repair question

Before sand blasting I need to repair some steel work on the frames. Do I disconect the negetive terminal from the battery before welding.

That's the safest way. Sometimes you can get away without disconnecting the battery, other times repair/replacement of the electronics gets VERY expensive if you don't disconnect. Other tips:

1) Keep the Grounding clamp of the welder as close to the work area as practical.

2) If working near radios, RADARs or their antennas, remove the radios if you can; or at least disconnect their antennas.

3) If the pieces to be welded do not have a short (approx. 1 Ft.), solid electrical connection between them, then BOTH pieces should be connected to the welder Grounding clamp.