What sort of preparation do they recommend?

Thank you for your question. We apologize for the delay in response. Be sure surface is free of all dirt, mildew stains, loose wood fibers and other foreign matter. Surfaces must be sound and free of standing water. Remove loose splinters and replace severely damaged or rotting wood. Prepare previously stained or weathered surfaces with Cabot Problem-Solver Wood Cleaner, Wood Brightener or Wood Strippers. On severely weathered surfaces, sanding may be required to completely remove all loose wood fibers Sincerely, Alex @ Cabot Customer Support