Professional plasterers will tell you a good chunk of their work comes from sorting out botch-job attempts at plastering walls or - even more calamitously - ceilings. So, we'd advise you to limit your repair efforts to hairline cracks and small holes and to seek professional advice for any walls where the plaster is "blown" (has come away from the wall behind) or damaged. Unless you fancy taking a course in plastering that is ...
For the jobs you do attempt, bear in mind that it's going to get messy. By the time you've cleaned out the hole, applied filler and sanded it down, you'll have created a thick layer of dust, so put down a large dust sheet before you begin.
Your filler options
Ready-mixed fillers: these are, of course, very convenient to use but they do have their limitations. They harden by evaporation and shrink as they dry out, so if you're using it to fill a deep hole it can take an age to set. And, when it finally does set, it may crack and end up not completely filling your hole, which is far from ideal ...
Powder fillers: these are mixed with water and harden by crystallisation and so set quickly right to the back of the hole. This makes them more suitable for filling deep holes. Unlike plaster, the powder left in the box will still be usable when you find another hole to fill months later.
Plaster: very economical, and therefore more suitable for large holes and, of course, entire walls. Some plasters (such as jointing compound) are easy to sand when they dry, so you can achieve a fine finish without having to learn difficult plasterers' techniques. Plaster will perish after a few months in storage.
Lightweight fillers: non-shrinking, ready-mixed micropolymer fillers (such as Polyfilla One Fill), are very convenient and can be used for medium-size holes, but the finished surface has nothing like the hardness of other fillers, so the finish will not be as fine.
Caulk: this is flexible acrylic mastic (such as Painter's Mate), and the best thing to use for filling any gaps that may appear between your skirting board and wall, your doorframe and the wall, or at the edges of shelves that are going to be painted. Because of its smooth plastic texture, however, it will stand out like a sore thumb if used to fill holes in the middle of walls.
Wood fillers: there are two types of wood filler: ready-mixed and "two part" fillers which comprises filler and a chemical hardener. The hardener smells revolting, but is a must if you want to fill anything bigger than a nail head, as it sets hard in minutes and can then be sanded smooth without the usual irritating wait.
Quick filling tips
Don't forget paint can be used as a kind of filler and that small cracks, especially in wood, can often be painted out by simply going over the area with your paint brush a few times. Just double check the paint doesn't start dripping.
Shove scrunched-up newspaper into deep cracks to give the filler a base to attach itself to. With larger holes, build up the filler layer upon layer, leaving each layer to dry before adding the next.
To plaster over small holes, first scrape off any old loose plaster with the edge of a scraper or filling knife. Then use a damp paintbrush to wet the hole. This nifty trick helps the filler bond to the surface. Now squeeze some plaster directly into the hole or on to your filling knife. Paste the filler at right angles to the hole, pressing the plaster firmly into place until it is raised slightly higher than the surface. When dry, sand the surface with medium-grade abrasive paper wrapped around a sanding block.
How to ...
Repair a hole in plasterboard
Tiny holes in plasterboard can be fixed with filler, but for holes bigger than about 12cm you'll need to fit a plasterboard patch first, so that the filler doesn't just fall embarrassingly into the cavity.
1. Start by neatening the edge of the hole with a craft knife. Using an offcut of old plasterboard, cut a patch that you can fit through the hole but is larger than the hole.
2. Before inserting it, drill a small hole through the middle of the plasterboard patch and thread a good length of string through from the back of it. Tack the string to the back of the patch with a nail.
3. Now apply undercoat filler around the front edges of the patch, and insert it into the hole (while holding on to the string, obviously). Pull on the string to bring the patch into place - the filler will make it stick to the wall, obscuring the hole.
4. Now you just need to fill the recess of the patched hole to make it flush with the wall. Holding the string taut, add more undercoat filler to the hole. To keep the patch in position as the plaster sets, stretch the piece of string taut and tie it to a chair or the wall opposite. When the filler has dried, cut the string flush with the wall and apply your final, finishing coat of filler.
Patch a lath-and-plaster wall
Providing your laths (the horizontal wooden struts to which the plaster was originally secured) are still secure, you can fill small holes in old lath-and-plaster walls in the same way as an ordinary plaster wall. However, broken laths may be the cause of your broken plaster. If this is the case, you'll need to reinforce the new plaster by placing a layer of metal mesh under your new plaster.
1. First, remove the debris from the edge of the hole until you reach a point on the wall where the plaster is still securely attached to the laths. Cut a piece of metallic mesh to the exact size of the hole with tinsnips (hand shears used for cutting metal).
2. Now put the piece of mesh in place and secure it around the edges with a plaster undercoat. Don't nail or staple it or you risk causing more damage to the laths and surrounding plaster.
3. Dampen the laths with a spot of water and apply your undercoat using a plastering trowel. To make sure it sticks fast, force the plaster in between the laths. Using the edge of your trowel, score the surface with a diamond pattern so you can identify the exact shape of the hole when you come to apply your finishing coat. Leave to dry before applying the final coat.
Plastering pointers
Only use undercoats of plaster if you're filling a hole more than 5mm deep. If you are patching a hole more than 12mm deep, you will need to build up multiple layers of undercoat: make sure you scratch the plaster in between each layer; this will form a "key" which gives bite for the next undercoat of plaster to grip on to.
After you've chopped your hole clean to the brick or blockwork and brushed away all the dust, paint PVA glue on to the area, mixed 5 parts water to 1 part glue. This coating should prevent the dry, old plasterboard or blocks drawing all the moisture out of the new plaster, leaving it as cracked as it was before.