I found a trick to use with sludge caked on the inside of valve covers by accident. I would lightly wash the inside of the valve covers with a paintbrush and gas, wait five minutes and do it again. Drain all the liquid off, and lay the valve cover out in the hot sun for about four hours. After that, I would lightly tap the cover with a hammer. All the sludge, even the stuff behind the baffels, would fall off and you would be left with bare metal. No scraping or other work. The gas dries out the varnish and it shrinks and cracks in the sun. Any disurbance then makes it fall off. Probably works with just smoothe surfaces.