In an ideal world this would never happen, but I sometimes find that I cannot cut away all damaged metal and must deal with blow-thru when welding patch pieces in. Do you have any tips/suggestions for minimizing blow-thru (other than cut more metal away)?
If you find yourself in a situation where you’re having trouble blowing through, you can clamp a 1/16″ or thicker piece of copper behind the seam to be welded. The copper will keep from blowing through and will not be permanently welded to the weld itself. Sometimes the copper sticks a little but a little tug or twist sets it free.
There are also copper welding spoons available to hold, clamp or have an assistant hold on the backside.
There is also something similar available from Harbor Freight and others… in a pinch, I’ve used plumbing connectors and copper pipe flattened out. Copper does dissipate heat quickly so it’s helpful to know that copper will get hot… if you fab your own use a vice grips or similar to hold it.