Form-a-gasket is a rigid sealant, it is not designed to be used to seal soft rubber gaskets to hard surfaces. I've never understood why some people try using a gasket (RTV, or form a gasket, or anything else) in addition to the rubber gasket. The RTV at the joint locations is to fill any gaps, that's all you need. The rest of the head is flat, another layer of gasket is just creating another failure point.