alsphaltum is basically a varnish made from bitumen,...it is basically refined tar roofing cement if you strain it and thin it properly the old fashioned roofing patch should suffice,..check the contents on the label and be sure it is traditional oil and asphaultum based,not the newer water based and recycled tire rubber latex substitutes,..
The technique for doing shades with it are basically like tole painting,..you wet one side of the brush with the asphaultum and then pallette the brush with some clear till you get an even stroke that is dark on one side and almost clear on the other,...done properly it is an art form in itself,...
edited to add,...you'll have to clear over it when you are through,...true asphaultum varnish will not dry hard enough to stand up to any weather,..ever see an antique chair with a finish on it like a thin layer of tar? thats asphaultum varnish,..it will stay sticky for hundreds of years,...