I first used HWFF's 'foam coat' and was very unhappy with the result. Yes, the stuff was strong, but it was next to impossible to work with. Sets way too quick, and goes on as thick as mud. I'm not sure how that styroplast stuff will be. Might be better, I don't know.... but i'm pretty happy with the results I have gotten from demandproducts.com.
I have been told by other guys constructing similar signs made from foam that these will last many many years. I have not see for myself first hand, but the everyone I have spoke with swears by it.