There is nothing wrong with backing up the threads with Marine Tex. I've repaired quite a few with that method and never had one returned because the epoxy failed. Sometimes, to save an old or valuable main casting you do what you need to.
I've seen (and even paid for) machine shops bore out the entire deal and press in a new threaded seat. 9 out of 10 times that ruined the castings as a pressure test showed tiny streams of bubbles around the pressed in area.
A heli-coil is not a good repair in that area as it removes most of or all of the sealing surface you you'll end up gluing it in anyhow, or trying to find and use a wider and thicker gasket under the seat. That throws off the float geometry and not really a good repair....IMHO.
I've done the hand-lapping deal using a seat with no threads, hit and miss depending on how deep the pitting or damage is. A good tool to turn the seat is a long tapered punch that fits it pretty tight, then you can two-hand it like a lapping tool and cut it much faster than turning it with a screwdriver...........Cliff
