Undercoat for bathroom walls where there was mould

[torqueofthedevil]

Due to poor ventilation / condensation in a shower room (extract now fitted) some mould formed on walls / that’s all been removed but it’s left some slight staining. Don’t really think it’s necessary but wanted to put an undercoat on top of the existing paint before the new top coat.

Don’t need a really thick mould specific stain locker but just some general thinnish stain block. Any suggestions?

Saw somebody recommend Zinsser BIN somewhere else which I think is a great product but overkill for something like this.