Can this be used on foam backer board.

Hello, Thank you for your inquiry! Since I don't know which foam board you are referring to, I will say that, in general, they don't recommend mastic for adhesion. My suggestion is to call the board manufacturer and confirm with them their recommended installation product; most likely polymer-modified thinset mortar. Please call our Product Support line at (800) 992-6273 if you wish to discuss this further. Product Advisor #9