I like to use brush-on Permatex Super 300. The cup type freeze plugs with the wide sealing lip don't really need a sealer at all. The flat ones that seal by hitting the center with a hammer to expand them after being placed in the groove in the block are the ones that can benefit from a little non-hardening sealer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!