Best sealing valve cover gaskets

Try the straight edge on the head surface too. Could be the casting is less flat than normal.

I have the engine out of my car and pulled the valve covers yesterday. Cast aluminum Mopar performance with cork FelPro gaskets. When I put them on last year, I used RTV to adhere the gaskets to the cover and lightly tightened the bolts and let it set. Next day I torqued them lightly, and repeated the following day until the gasket stopped compressing and the bolts held torque. Fast forward to yesterday, two bolts on each side were "loose" or just over finger tight, yet I had no leaks. Process and decent parts will bring success.