If you have a well-stocked art supply nearby ask to see some of their Chartpak products. Graphic artists and cartoonists use these. Some of the products are a very thin black or dashed line with a sticky back (sort of like Scotch tape). Apply it to a ground glass to describe various outlines. When finished, it is easily removed. A similar product is made for white boards.

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http://www.magnatag.com/page/TP/supp...Fenm7AodTBgAog