Valve Covers (not a perfect seal)

Check around the bolt holes for cracks. I've run across lots of valve covers that had it's retainer bolts overtightened to the point that the captive washers cut through the cover. If they aren't cut, I like to hammer flat the area around the bolt holes, install new gaskets, and torque to specs. If sealer is needed, I like to use CopperCoat. I'm not a big silicone fan, but, RTV Ultra Black is another option that works well. Just don't get carried away with it.