I use a product called Structo Lite I get it at Home Cheapo in the drywall products. it is used for the brown coat for plaster . It is cheap and goes along way. pre wet the plaster cloth before appling so it doesn't suck the water from the Structo Lite. It comes in gray color and sets up in about 30 minutes. just when setting up you can scratch strata into it. It is the easyest way that I have found to do land forms. it also will go directly over foam, pink, yellow, blue or white it doesn't matter. It takes paints, dyes and stains really good. Give it a try and let us know.