This isn't just a dinghy question, but I thought this is the best place to ask.
My 1968 half tonner (cold moulded oregon hull, ply deck and cabin) is suffering from what looks like mildew and water stains in the grain of the ply on the cabin sides. One (a replacement cabin side) was only very lightly epoxied before they went in and it degraded over time, the other side is just varnished and isn't suffering as badly. Yes, I know it shouldn't have this problem, but the poor old Firewood Flyer is looking a bit sad these days.
I can't sand it back, because (a) it doesn't remove the marks and (b) I sand through to the next layer of ply. This problem is worse on the side where I did some replacement using modern ply, the old Brunzyeel is significantly better.
I've tried bleach...no dice. I'm also facing a similar problem on the old Moth, and with 1.3mm or so deck sanding once more is not a good option. Any other way of removing water stains and mildew? Or would a dark stain cover it up? I don't want to paint largely because nasty things seem to happen unnoticed under paint.