Pinhole Remedy After Welding Sheetmetal Patches?

TrailBeast said
Well that would have been a real problem when I welded up a big hole in a trans pan.
Cleanliness, temp, wire type and shielding gas is the solution.
When I weld something like that I put gas or lacquer thinner in it afterwards to check for leaks, since they both will show any leaks right away.

Exactly. All kinds of stuff is welded that has to be air tight, fuel tight, even pressurized at high pressure,. Fuel tanks, propane tanks, oil pipelines, etc, it's a long list. Pin holes are not inevitable.

sireland67 said
Just put a thin layer of fiberglass resin, paint it on with a small brush.
It will seal any pin holes that you cannot see, and it will not react with metal.

That's exactly the point. It doesn't react with the metal. It literally will not mix. And if (and WHEN) it leaks, moisture gets behind the repair and it rusts out. The metal on these cars flexes, the resin will separate, and the repair will fail.