Mary Kate On and Off? (Ignorance on) What is 'Mary Kate On/off?' Is this something from West....or from anyn other hardware store???
It's something you will find at WM and perhaps other boating stores. It works very well to clean up gel coat, I have never used it on my sailboat, but my fishing boat is not bottom painted and on/off will remove any slime/stain/organic material and leave it nice and white. I think that it may also remove any wax on the area applied to as well, so keep that in mind.
The stuff is very caustic, wear glove, I use a paint brush to apply... it works right before your eyes. I would need to re-read the label, but I would think that this is something that you do not want to apply when your boat is on the water and spray off into the water... I have several thousand oysters growing in an oyster garden off my pier and wouldn't want to kill them...