Plasterboard below bath line, do I need to prime?

[hurders]

Perhaps a daft question but here goes,

Replacing my bathroom suite and retiling. I've got all the old tiles off and the bath out. Walls are standard plasterboard and skim with the exception of the are below the bath line which is bare plasterboard which has a few dodgy black areas where water has leaked down the edge of the bath. Before I put the new bath in should I treat the bare plasterboard to anything? (its not really so bad that it warrants replacing) Should I prime or seal it with something? (and if so what should I use) Or should I just leave as is bearing in mind its completely below the bath line and not going to be supporting anything?

 

[iaith]

In an ideal world you would replace the plasterboard with cement board. the black marks below the bath line indicate the seal between the bath and wall has broken down. the plasterboard may look ok but may well be rotten, and a new bath might not stick well to rotten plasterboard. If you decide to keep the existing wall , I wouldn't worry too much about priming below the bath line but I would check for leaks as you don't want the floor that the bath is sitting on to rot.
Good Luck.