detached paint

Not disagreeing with anything said so far, but it sounds like the ply could be de-laminating. If this is the case you need to try and diagnose the cause - poor quality ply, void in the inner veneer, rot,..... If it's restricted to a small area, and not due to rot, you may be able to repair it by working some glue ( I would use epoxy with thickening additives) between the veneers. As this area is to be painted, you can fill in spaces made by missing veneer with epoxy. If it's a larger area, or due to rot, you may have to cut out the bad area of ply and fit a new patch, or even replace a whole panel.

If some of the outer veneer has come off (as I understand has happened from your first post) then you should be able to work a little epoxy glue in between the outer and inner veneer layers at the edges where the piece of outer veneer is missing. I suggest using a thin sharp knife blade to do this.

Epoxy would not soak in through the veneer, but painting some onto the outside might help strengthen things a bit if the ply is dry and there is not rot.

I use West epoxy http://www.westsystem.com/ss/ , but I'm sure other brands like SP are just as good. I would use colloidal silica (West ref 406) and microfibres (West ref 403) as thickening agents to make the epoxy a great gap filling wood glue. You won't need much epoxy, but even a small pack plus these two additives may appear rather expensive, however it is a good investment - epoxy can be used in so many ways, for taping joins, wood glue, filleting, filling and fairing.