I have ran into the same problem with galvaneal sheet metal. Using it for compartments in firetruck bodies.
The solution we came to was to lightly sand the edges that were to be welded, this takes off the coating in the weld area and makes for a good weld. The coating burns off during the weld...
I started working on a Toyota pickup with an aluminum head.
The eng. head has 2 stripped out bolt holes for the exhaust manifold.
What is better? Helicoils or re-drill and re-thread?
Or is there something else that can be done without welding on the eng.head?