The age old question of what material to use, Moisture resistance MDF, something melamine covered, aqua board ? There is a shower over the bath with a shower curtain. The vanity unit its going next to is gloss white but not bothered about finish match more about not swelling and having to redo it. Have already googled it to death with various results.
Apologies but I'm gonna disagree. Why would you use probably the most absorbent wood (if you could call it that) and try to protect it? The slightest water ingress and it will swell. I would use ply wood at the very least but would prefer marine ply if budget allows; You would still need to put protective finish on it.
There are UPVC boards you can get which look alright in some bathrooms. They're not overly expensive. Try Eurocell or somewhere similar.
As @WillyEckerslike says, you could create your pipe box out of uPVC facia board, and that would last you forever given that it won't ever see direct sunlight. You could drench it all day every day and it would still be fine in 20 years time. Frankly it wouldn't be my choice of a finish for a bathroom, but then I'm a bit of a traditionalist and I box my pipes in and then tile and grout the boxes.
Will go that route I think, then I don't have to worrying about sealing it etc. I would of rather it be tiled but it wasn't possible in this case. Thanks for all the replies