Has anyone had an oil leak from the valve cover gasket of these plastic valve covers, and then "fixed" the oil sealing problem by sanding down the mating plastic surface so it returns back to being flush with the cylinder head surface so it is then able to maintain a good oil seal after installing a new gasket?

Using my feeler gauge with the cover laid flat on a smooth flat surface, I found that between any two bolts of the original OEM valve cover the thermoset plastic had receded away from the cylinder head surface by approximately 0.63 mm, the thickest feeler gauge in my set, creating an arc in the plastic molding, so going around the part there are a series of such arcs between all of the bolts, which by their nature had to remain fixed to the cylinder head surface. Figure that must be what has been happening to the plastic in all those valve covers that have been replaced that I have seen in so many threads here. Figure it's a plastic aging problem due to use of the plastic over a long time with the engine going through so many heat cycles.

Measured the Fel-pro gasket I bought for the purpose and found that there might be enough extra "rib" (the part of the gasket that fits into the canal) to permit the plastic cover to be sanded smooth enough to achieve a good oil seal with the new gasket, but have not done the experiment yet. I know the plastic might have reached some kind of limit, so this might not work as I imagine, but unless someone tries, we won't know it the life of the plastic valve cover might be extended a bit.