The only time I use a sealer on a head gasket is when it's a single-layer steel shim type of gasket. The composition-type, or the "no retorque" gaskets already have a sealer built in. The best head gasket sealer I've found is a brush-on liquid, Permatex Super 300. It's a thick, almost black liquid, not the runny 3D type. It was developed for use with parts exposed to aviation gasoline back during WW II. Hard to find, but good stuff!
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!