Basics of surface preparation
In all aspects of DIY the quality of the final finish will always depend on the amount of preparation that you do. This is no less true with surface preparation before painting or wallpapering. As this site tends toward basic DIY we'll cover just the preparation needed before painting and wallpapering walls, as those are the basic building blocks of home decor.
We'll start with the walls first. It's not necessary to remove all the layers of paint before painting over or wallpapering, but if a room has been papered then that has to go. If there's no wallpaper to remove you can skip the next few paragraphs.
Wallpapers strippers can be hired; they use steam to loosen the glue holding the wallpaper to the wall so that it can be peeled off. This is well worth doing if you have many rooms to do, if you are only doing one room you may find it cheaper to remove it manually.
Much of the paper may come off just by peeling it back, depending on how long it's been there and how good the job was in the first place. The remaining pieces can be scored with a sharp knife (but be careful not to go through and scratch the wall) and then warm water applied with a large sponge. After a few applications the glue will soften and you can remove the paper with a scraper. Now you will have a wall that's ready to be cleaned and prepared.
Filling Cracks and Dents
The next step, and welcome back to those who didn’t have wallpaper to remove, is to get the surface smooth, dry and free from blemishes. Start by brushing the wall down to remove and loose particles so that you can see what you are left with. If there are any cracks make sure you get all the loose dust out of them.
We'll tackle those cracks now, along with any other dents or damage that needs filling. Get a ready-prepared filler or mix some from paste, following the manufacturers instructions. Spread the filler into and over the cracks, smoothing over with a wet palette knife or spreader.
Push the filler into the deeper cracks and don’t put it on too thickly. If there are deep holes build up thin layers rather than trying to fill them all in one go. Leave the filler slightly proud of the wall surface so that when it is sanded back it will blend into the surrounding wall. Sanding can be done after the filler is dry.
Sanding Down the Walls
Sanding can be done with an electrical sander or by hand. Hand sanding is more tiring but a lot less messy. If you go the hand route, attached the sandpaper to a flat wooden block and rub the walls with that. Don't be tempted to use your fingers as you will make grooves in the wall.
Go all over the wall, not just the areas where you have applied filler, as you will want to rub down any high spots too. If the wall is bad then use a rough or medium grade of sandpaper first and then follow up with a finer paper. Otherwise just use the fine paper from the start.
Final Wash Down
Finally brush down again to remove all the little grains that the sanding has left behind and wash the wall, either with warm water or a sugar soap solution (you can buy this from DIY shops) followed by water.
If the sponge or cloth you are using to rub down is still looking dirty, keep washing until it comes off clean. Don't rub too hard though, as you will be removing old paint which will make the cloth look dirty and you'll never stop.
Take Pride in Your Efforts
Of course, this is all very tedious and boring. Unfortunately, if you want the best finish, one you can be proud of, it has to be done. But it will be worth all the effort once you see the finished job.