I used JB Weld to fill in a crack on my dad's valve cover on his 235, then I prepped it and painted it with rattle can engine enamel.
I filled in a old mirror hole with JB weld on one of the doors on my '65... then sanded it down. It is still in there.
Putting on JB weld is like prepping for painting, just make it clean before ya do it.
-W