I like the blue sealant for what it is, but here's the method I have used on valve covers for nearly 20 years and have NEVER.... check that again... NEVER had one leak....
I clean the inside of the cover completely with brake clean (no oily residue when it evaporates) Let it dry completely. Next, run a coating of Permatex 3H, or Indian Head Gasket sealant around the lip of the cover. Main thing is you want something thin that will grab the gasket.... I have never been a big fan of the Gorilla snot doo-doo.... Put the gasket in the cover, pressing it in, so the sealant grabs the gasket well... Let it sit for a while.... Then, once it's stuck nicely, not before, smear a really thin coating of some kind of grease on the exposed part of the cork gasket. I use the leftover red high-temp stuff i use for bearings for this part... Then, put them up on there, pop the bales back on and you're good. The grease helps them slide around a little so they don't pull away from the cover.
The godfather used to roll his eyes and tell me to put them on bugs dry, but that never worked for me for some reason.... So I guess there was SOMETHING I managed not to pick up from him over all these years....
For what it's worth, I am seeing more leaks due to the gaskets pulling up, like yours in the pic here lately... If i had to guess, maybe something to do with the material they are made from which has changed over the years....
