Can this be applied to a deck with paint already on it without having to sand it off?

Thank you for the question. We apologize for the delay in response. You will need to prepare previously stained/painted 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

Remove any loose paint, but you don't have to sand off old paint