Plasterboard over plywood or skim the plywood

[Diymumma]

I am removing some tiles in my bathroom, behind there is plywood. I have been searching what to do with it because I want to paint it, but I am confused either I skim over the plywood or put some plasterboard over it and then skim. Looking for some advice on what is the best solution.
Thank you in advance

 

[Wayners]

You can't skim plywood.

Either elements board or plasterboard over.