How can I fix this

[ForAmberZebra]

I removed some of the tiles in my bathroom and encounter this. I want to paint, how can I fix this, especially the top part on the narrow wall? Am I better of removing the plywood and the "concrete/sand" and put plasterboard? Also what is the best product ready mixed to skim? Many thanks in advance

I can see why it was tiled!
I’m going to, gently, suggest that if you are unsure about it, it’s likely not going to be a brilliant finish if you do it yourself tbh.
I mean, I can polyfilla pretty big gaps and holes so smooth they don’t even need sanding, but I wouldn’t attempt skimming an area that size. It would look awful if not done smoothly. Is getting someone in not an option at all?
Failing that, B&Q often hire very handy people who happily share tips and offer advice on the best products to use.

Get a man in, a professional.

If you start on this yourself it's going to look horrendous.

There are roll on products that will give a good finish, but this is too rough for that. I'd be boarding over with cement board or water resistant ply/MDF.
That bit of ply in the first pic could be an access panel, so needs to remain accessible.

Thank you everyone for the suggestions. I will pop into B&Q and see if anyone can advise. I am normally pretty good at DIY and not afraid to try and fail it's all learning.

Yeah sorry OP I think it’s this ^^

Im a pretty confident diy’er myself but there’s no way I’d attempt that. Agree with the PP it’ll likely need re-boarding; you’re not going to get that surface/s looking any good without replacing or going over them.

If you're determined to do it yourself, I'd remove the boards on the narrow/box bit and put some water resistant plasterboard on it, and board over the rest of the walls too. Painting with bathroom paint will be ok on the walls that don't get wet.

Around the shower/bath area you'll need more protection than paint.

Yes that is what I ended up doing, the cement/sand was only there to make the width of the plywood underneath, behind the cement was a bit of plasterboard. I am getting some water resistant plasterboard tomorrow. Just working out if I add another plasterboard on the bigger wall if I then can hang a led mirror there without falling off 🤣. Thank you for the suggestion.

Get some long screws and make sure the wall plugs go into the solid wall behind and it'll be fine!

The problem is that I don't know the width of the plasterbord already there, I am assuming either 9.5mm or 12.5mm just have to try and test and pray for the best 🤣. Thank you much appreciated