Preparing a wall for painting

I've just stripped down the walls and ceiling in my bathroom and now need to make good before I paint them. There are some minor imperfections as well as one or two cracks which I was just going to fill and then sand. Then I saw theis Polycell Smoothover that looks like it will give me a good finish. Is it worth using this stuff? It looks difficult to apply. When I saw it I thought it just rolled onto the wall but after reading around it sounds like it goes on like plaster, with a trowel or similar tool. Maybe I could use the Polycell Base Coat or Problem Wall Treament instead?

If I do decide to just paint straight onto the plaster, do I need to do anything to seal it against moisture? I was just going to miscoat with thinned emulsion.

Cheers for any help.

[EDIT]: I'm using a silk emulsion as I read this was best for bathrooms. Can I use a thinned coat of this to seal the plaster?

OK, no replies but an update anyway. Once I got the radiator off the wall the plaster and much of the board was crumbling. So I've decided to get new plasterboard put up on that wall and have it skimmed. I'm not sure whether or not I should just get the rest of the walls skimmed. Would the plasterer have to remove the existing plaster or could he skim over? Some of the plaster on the ceiling has a thin coat of paint on it.

It is all down to cost, in terms of whether to take down all plaster and redo. Me personally, i would scrape off all loose plaster and patch up, then apply multi-finish plaster. It will be a better job at the end of the day.

Cheers, I'll see if that can be done. I think I'd like a whole new surface to pain on. I was just going to get one wall done (the knackered wall) but I might as well get it all sorted while I have the chance (and the will!).

Acrylic eggshell is a good finish andsimilar to "kitchen+bathroom" paints