>>>*Nearly all sealers work on the same principle, just hair in the bathtub drain scenario.

In high stress areas, the fix is temporary at best.

There is one type of sealer that I know of that undergoes a catalyst type reaction under heat and sets up like metal. Many machine shops use it when they remanufacture an engine block, they just drop a cube of it in the water jacket. That is called "Magic Seal" and is mostly sold to people in the industry, it isn't commercially available, partly for toxicity reasons.

This works very well for cracks and things like that. I have sealed up cracks in heads that were inches long and pouring coolant.

Even at that, it is still temporary and will eventually fail. But I did seal up a cracked head in my own Oldsmobile 98 for over 100,000 miles. Surrounding mass of metal to help support it is why.

A head gasket?

Nope, I would not even suggest trying a sealer. Best to get the wrenches out....*EB