Answer: I’ve often had problems getting chrome valve covers, oil pans, and rear axle covers to seal properly…until a wise old mechanic told me the reason and I’ve never had the problem since.
The problem is usually the chrome itself. Sealers do not like to adhere to chrome, and chrome is so smooth that gaskets will tend to “creep,” reducing the effectiveness of the gasket. The solution is to remove the shiny chrome surface in the areas that you want to have a good seal. The quickest way to do this is with a wire wheel, Dremel type power tool, small die grinder, or small electric sander. Sanding by hand works just as well but takes longer.
Whether you are using a gasket, a “gasket maker” in a tube, or both, this will usually solve the problem.
There are a few other common mistakes that I see that will often lead to a leak. If using a chrome valve cover, or any other tin valve cover, be sure that the gasket surface is flat by placing it on a flat surface and looking all around the edge. If a chrome valve cover has been installed and removed, I can almost guarantee that it will not be flat. The areas around the bolt holes will have been pulled down by the tightening of the bolts. This must be corrected, or it will positively leak when reinstalled. The easiest way to correct this is to place it on a flat surface and bend the tin back in the opposite direction by placing a piece of wood against it and tapping it with a hammer. You will have to do this around each of the bolt holes.
If using a gasket, I find it best to first affix the gasket to the valve cover by using a small amount of high quality adhesive such as Permatex Ultra Blue RTV Silicone. Be sure to apply the silicone all the way around the bolt holes in the valve cover, but don’t use too much silicone or you will have high spots and low spots along the gasket when the silicone dries. When the silicone between the valve cover and the gasket has dried, apply a thin bead of silicone along the edge of the gasket that will contact the cylinder head. Install the valve cover on the cylinder head being careful not to over-tighten the bolts. Over-tightening the bolts is a sure way to create a leak.