There is a kind of way to pot a pup lightly, but it is completly different to all things said here.
There are some points of a pickup that are prone to squealing:
First: the cover. This could be solved by a dab of silicone or tape inside the cover.
Second: the bar mag. Drop some wax on it and slide into the pickup and tighten the base screws. While heating the baseplate, tighten it more to glue the mag to the baseplate and the bobbins.
Third: the coils. I leave them alone to get the alive feeling at certain point of gain.