With most cam covers, overtightening can distort it and cause leaks. Any leak can often result in tweaking the screws, later making things worse.
First thing to do is check the cam cover is flat. If it isn't sealing it will be difficult even with lots of goo.
A good quality gasket should do the job. If only! Usually, a new gasket and a thin smear of sealant will seal it. Don't overdo the sealant, as too much can make things worse. If using a gasket and sealant, fit it almost immediately after the sealant is applied and tighten it down gently. Leave it to set, then tighten fully.
If you decide not to use any gasket, you will need to create a bead of sealant that protrudes out of the gasket groove by the same amount as a gasket would, or a little more. But not lots more. Leave this to partially set. Should be touch dry, but prodding can break the surface. Determine this by placing a similar size bead somewhere that can be used as a test. Once touch dry, place cover on, put bolts in, but spin them down only until they touch, almost no pressure. Now leave a while longer for it to bond and set some more. Once fully set, and therefore flexible rather than smearing, you can tighten it down. Again use your test sample to determine this.
This procedure creates a gasket out of the sealant, but effectively glues it to the surfaces. If it fully sets before you place it on, you've just created your own gasket and not really fixed anything. If you tighten it too soon, you'll just squeeze it out, so you'll effectively get a metal to metal joint with a tiny bit of sealant in it. The rest of the sealant will be messy outside, or squeezed inside, ready to set in your oilways. So pop it on whilst still slightly sticky, allow to set fully so it is flexible, and you should get the best of both worlds.
Of course, both surfaces need to be clean and dry. Any oil residue will prevent it sticking. But you knew this already.
All takes a lot longer than just popping a new gasket on, but if then fixed until you next need to remove it, it'll be worthwhile. Rover Maestro/Montego 1.6 'S' series engines had very leaky cam covers. This procedure fixed them.
