any non metal chemical you put on will eventually shrink and crack.
the only sure way of doing this is to weld them shut.
to do it right, it takes a mega ton of patience.
getting behind most of the holes is a two or more person job and some holes are impossible to get behind with a piece of carbon or copper.
What I did, arc the welder across the holes and create "bridges" with the wire. then turn up the heat and fill it in.
The hard part isn't the welding, its the grinding. When you grind the weld down, it will heat and warp into a low spot so fast that you will almost see it sink in.
Grinding it is where the patience really comes in. You might even get a finer grit grinding wheel and just take a little off at a time .....not letting it get hot at all.
Done right and you can make it look like a hole was never there.
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