Plastering Beginner's Guide
The Plastering Beginner's Guide is designed to help get you up and running.
This article is not intended to be a complete treatment of plaster or plastering, rather it is meant to be a simple, straightforward and easily digested starting point. To read full detail about this subject, see Plastering.
Base coat, undercoat, scratch coat
It's function is to give a flat surface with the correct direction (eg vertical) and to even out undulations on the bricks or blocks beneath it.
Generally around 1/2" or 1cm thick but can vary quite a lot.
Skim coat, top coat, finish coat
Hard and smooth fine layer to provide a good surface to receive paint and wallpaper. Generally a few mm thick, can be quite thin (eg 2mm) for a repair coat. Two coats are required in immediate succession for a correct finish. The first coat is to proved a level finish upto the tops of any bumps in the undercoat. The second coat will allow a polished smooth surface to be achieved, which is not possible with the first coat due to the undercoat showing though on the high bits.
Plaster must be fresh (it will usually have a Use By date on the bag) and kept dry and away from damp. If storing on a concrete floor, it should be raised off the floor or stored on plastic to avoid damp getting in.
Old plaster can occasionally find uses in small patching works where it is actually desirable have have a fast setting time, but it would be unwise to use old plaster on a big job like skimming a wall.
Preparation of surfaces
Needless to say all surfaces should be sound and free of grease and wax before attempting to plaster. Loose skim coat may be easily knocked off with a chisel, a small or medium crowbar or any other suitable tool. It is worth soaking the area with dilute PVA (about a 5:1 mix) to bind any remaining minor areas of flakiness. Large areas of loose undercoat should of course also be removed, but very small areas will probably hold together well enough, especially if soaked in PVA.
One method of locating loose areas quickly is to tap the walls all over. I found a 19mm (ie largish) ring spanner made a good sounding tool. Some people can do this, some can't. With practice I was albe to determine the difference bewteen sound areas, loose paint, loose skim coat and loose undercoat. Mark the areas with pen or pencil then go round knocking off the loose stuff.
Any hairline cracks should be taped over with scrim , this can be stuck on by using a small mix of plaster troweled along the crack and the scrim tape pressed onto this. Self adhesive scrim is no good on its own as the surface area for the adhesive is next to nothing. Scrim tape should be used where there is a change in the substrate material.
Should be left for 24 hours before application of plaster.
Ideally, this should receive the skim (top) coat on the same day, whilst still wet. Failing that, it should be coated with a dilute PVA mix prior to skimming.
These should be coated with a dilute PVA mix (say 3:1), which may be applied with a cheap roller for speed over large areas. Wait until it's gone tacky (not dry) before skimming.
How to skim
Plastering, like anything else is easy if you follow a tried and tested method:
Get the wall ready first, don't start mixing until you have done this...PVA over everything, except plasterboard, you can apply the skimming while this is wet or dry....if the wall is p-board, get it scrimmed up all along the joints and make sure no nail/screw heads are proud.
- get a coat of plaster on the entire wall, don't bother about any marks, lines or anything else, just make sure the entire surface is covered fairly evenly (even as in the same thickness - don't have it a mm in one part and half an inch thick elsewhere, unless this is unavoidable.)
- Wash your trowel, bucket etc and clean the mixing bucket and then have a brew.
- 10 mins later, do another mix, half the size as the first mix.
- apply this t-h-i-n-l-y over the now partially set first coat, you are only really using the new mix to fill any hollows and you will see that it is much smoother already than the first coat.
- wait 10 more mins when you've finished the 2nd coat, then with a clean trowel and clean water, splash a small amount of water on the upper left section of wall and work your way L-R, then the same at the bottom. (the handle side of the trowel should be kept clean at all times, if any bits of plaster are visible when looking down at the blade, clean them off, have a wet brush and bucket at all times when plastering).
- repeat number 6
Some people attempt to do large walls in one coat, but it's false economy, firstly, it saves them 75p worth of plaster, but they spend longer trying to get a decent finish on it, and secondly, because they think they are saving time by 'not doing it twice', instead they end up going over it a dozen times trying to get it right, whereas with the method mentioned above, it gets two coats, laid down once and finally polished, but there's less effort goes into those 'four times over' than 'one coat and struggle for hours'...the end result is a smooth finish suitable for painting, if the walls are to be wallpapered and it's your own house, you can leave the final polishing, but for the sake of an extra 30 mins easy work per wall, it's not worth leaving.
How to skim - variation
From personal experience of an absolute numpty beginner, based on Ron The Builder's How to plaster a wall
This may not be the professional way and there may be much scope for improvement, but I achieved quite passable results immediately. Not professional standard by any means, but good enough and hopefully practise will help.
- Follow above guide regarding preparation and PVA-ing over friable substrates (I use a 5:1 water:PVA mix) and further PVA-ing over the lot, if there's any paint to be covered (I use a 3:1 Water:PVA mix for this)
- Mix the plaster (I use a MultiFinish plaster, fresh as possible) until it's like whipped cream (a power mixer tool on a power drill or SDS makes this easy and avoids lumps).
- Apply plaster mostly evenly but don't fiddle with it. You have about 30 minutes to get this done before the mix starts setting.
- Have a brew - very important. At first, do not attempt to be clever and do anything else, other than wash your tools. Keep any spare mix on the board.
- Watch the plaster, occasionally poking it.
- When the plaster is becoming cheesy in consistency (another 15-30 minutes later, depending on how quickly you got the first coat on, going the way to Play-Doh or putty in consistency), take your clean trowel and flatten the plaster as well as possible, with the trowel at a shallow angle. This is much easier now it has set to a semi plastic state. Any obvious gouges or low spots may be filled with what's left on the board, but don't worry about absolute perfection. It will look not very good at this stage but don't worry. Do your best to get general overall flatness though.
- Clean tools and board
- Wait for plaster to become quite firm - another 15-30 minutes. It will still be damp, which is what you want.
- You may wish to gently trim edges of your back boxes with either a sharp knife or the edge of a small pointing trowel - it's easy at this stage. A good time to gently remove any excess that may have escaped round an internal corner.
- Mix perhaps half the amount of mix you did the first time, perhaps just a fraction wetter - but not too wet.
- Apply very thinly with a scraping action. You aren't trying to put another thick layer on - rather you are trying to get plaster into the hollows as defined by your trowel. Because the first coat is not fully set and damp, the plaster will blend surprisingly well (well I was surprised!)
- Do this over the whole wall - this will be a quicker session than the first coat. You can aim for general perfection here, but don't worry about the odd ripple the trowel leaves.
- Wash your tools and give it another 15-30 minutes until the cheesy stage is reached again, keeping your leftover mix on the board.
- Have another brew and watch the plaster.
- When it's got to the cheesy stage, flatten in any slight ripples or marks. If you discover a small low spot or a gouge, you can still work some leftover mix in provided it's no firmer than putty. A spray of water from a plant mister can help the trowel move nicely, but don't overdo it as the gypsum is very soluble at this stage and sloshing water everywhere can leave run marks.
- Trim any excess from internal corners, external corners and back boxes. Careful - pulling motions can dislodge lumps - try to cut into the plaster rather than pulling away.
- A 2-3" paintbrush dipped in clean water can now be used to gently run round all internal corners - this should blend the corners in quite nicely.
- Clean your tools and board and site and admire your work with another brew!
A plasterer's trowel is flat (ish) and rectangular with a handle centrally mounted on one face.
Trowels...you can rub the sharp corners off on a brick if they are giving you too much grief, just make sure there are no 'burrs' on the face of the trowel side...they only need half a dozen scrapes on each corner, just to take the sharpness out.
Another alternative, I recommend for beginners like myself is to buy a pre-formed stainless steel trowel, such as the Marshalltown Permashape. I used an old flat trowel and a Permashape in alternation and there is no comparison in the ease of use of the latter.
Hawk: Needed to load the trowel and to scrape the trowel off with. Available in metal and plastic.
You will need a bucket - something with a flat bottom and smooth sides and something to mix it with. This can be a stick, which will be hard work for larger amounts but is still viable for smaller amounts.
Or you can buy a power mixer: A helical mixer tool on a reasonably powerful electric drill makes a fine mixing device. Beware of overloading underpowered drills - you need to run it for a few minutes at a time. Real plasterers will often have a dedicated power mixer. The tool is much the same but there's a more powerful motor with a double handle for easier handling.
When mixing with a power tool, it is best to run the tool as slow as possible to avoid entraining air into the mix. However, with an electric drill, this may overheat the drill as it is providing a high torque at low motor speeds (the gearbox not being optimised for this). If signs of overheating are detected, the tool should be detached and the drill run on full speed with no load to provide immediate air cooling - don;t just set it down and leave it.
The "proper" power mixer avoids this problem by having more gearing down. This is a good use for those heavy (8kg) SDS drills that sometimes come on special offer from certain German supermarkets!
Whilst it important to mix well enough to avoid lumps of unmixed power, over mixing can result in the mix setting quicker than expected.
You must keep everything clean! Clean clean clean! Lumps on the board or hawk will break off into your mix and ruin everything. Crud on your trowel will remove any chance of a decent finish. If you have the luxury of a garden and a hose, simply take stuff outside and give everything a good blast with the hose. Gypsum is not bad for the ground and may even be good if you have clay soil. A hose is a very fast way to maintain shiny clean buckets and tools.
The presence of old plaster causes faster setting of the new mix and reduced working time.
If you don't have a garden area suitable to hose stuff down, then wash tools into a large bucket, barrel or any waterproof containment. The plaster will sink and the water can safely be poured off down the drain later. The plaster can be dug out, and along with any leftover mix, put into old cardboard boxes or used paper plaster bags, allowed to dry and taken with reasonable lack of mess down the dump.