I have old hardwood windows, currently done in a very dark brown Sadolin. I want to stick with Sadolin, but one of the nice colours of Superdec, probably a greenish/bluish/grey. Can I just sand off the loose bits, and paint straight over, or will the dark brown show through badly? If so, is it necessary to sand down to bare wood, or is there some kind of undercoat that would do the job? I'd hate to undercoat unnecesasarily, it's an enormous job as it is!