We used a well known overwintering product on our caravan at the end of 2017 and discovered when we washed the caravan the following spring that the product seems to be stuck to the surface of the caravan like a lacquer. We have tried normal cleaning products but don't want to use anything too abrasive and ruin the caravan - although looking at it you could say it's ruined anyway. We've tried Silky cream cleaner on patches and it seems to work but you have to put some effort in to have any affect. The caravan is ten years old. Any other ideas?