Depending on the state of the boards, what I do is to give the boards a minimum of 2 or more good coats of PVA glue.
It's ideal for prepping and sealing new and old plaster and plasterboard.
Of course if the boards are so bad, they will either have to be replaced or plastered over, but that’s another story.
I use a cheap paint roller and tray, one of those ones you get from B&Q for about £2 and some cheap paint brushes; just throw them away when you’re finished.
Depending on the type of PVA glue you use, dilute about 5 to 1.
You want the glue to be like water.
The 'value' type of glue is thinner compared to the better quality glue.
B&Q do their own brand of PVA.
It's a large 5 litre can for about £10
Do a trial first on a small patch and see if the glue is soaking in or just lying on the surface and dilute as required.
The idea is to seal the boards, you don't want the glue to stay on the top of the boards; what you want is for it to sink in and provide a key and sealer.
Don't be too enthusiastic in applying it as it will go all over the place.
Just paint it on at a slow and steady pace about one metre square at a time and overlap the edges.
The same way that you paint a wall with a roller.
Let one coat dry first and make sure you cover all of the boards, don’t leave any blank spaces at all, smooth out any runs and put a good blob into any gouges and cracks.
Use a brush for the boards at the ceiling and skirting and at the inside corner of walls.
After drying, you can then fill and prep the boards before painting.
Dab a good drop on top of any filling that you’ve done as this will seal the filler as well.
Some adhesive manufacturers say that you don’t need to seal wallboard before tiling but I prefer to.
I’ve used the glue on new boards as well before tiling.
After prepping with glue, I even go as far as giving plasterboard one good coat of bathroom emulsion before tiling, so that water in the tile adhesive does not get soaked in by the board.
This is probably overkill on my part anyway!
When painting, any good emulsion will do but you might have to give the boards more than one coat to get an even finish.
Watch out for the cheaper brands of emulsion paint as they dry they have a tendency to form hairline cracks.
Although you can paper if you prefer.
If the boards are not too bad, the best paper is blown vinyl, this will disguise any minor blemishes.
I’ve used blown vinyl on badly plastered walls and the only way you can tell about the plastering is when the paper is stripped off again.