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Summary: How to plaster the skim or top coat layer of plaster
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Top coat plastering, or skim as it is known is a very difficult job indeed. It is not difficult to actually get it on the wall or ceiling, but getting it flat and smooth requires a great deal of hard work and practice. As with every tradesmans job we suggest practice. Then more practice. If you get it right, doing your own skim plastering can save you a lot of money. If you get it wrong it can cost you a fortune putting it right. Buying (and being prepared to throw away) a sheet of plasterboard and a bag of top coat plaster will cost an extra £20.00 or so but will be worth every penny.
The main thing that DIY'ers are unaware of is just how quickly top coat plaster goes hard. As soon as the first coat (we always recommend two coats) is "laid on" it is usually ready to be trowelled up and the second coat applied. Skimming is as much about timing as anything else and trying to get a polished finish too quickly, or too late, can result in a horrible job.
Before attempting the above, we suggest you first mix up an eggcup full of skim. Place this on an off cut of some kind that you have prepared with some pva. Spread it out and mark the time it takes to go firm but not hard. This is the point at which skim can be made smooth and the trowel marks will disappear. Once skim is hard the only way of smoothing it is by sanding. As with all of our projects, the money spent on practice is a tiny amount compared to the amount it takes to put a job right. “Stop end” beads can be purchased from the tool store below. These are galvanised or stainless beads, which can be fixed to the surface and allow you to divide your plastering area into smaller, more manageable sections. Their flat top gives you an edge to work to and finishes flush with the surface of the plaster. When decorated they are not visible.
Mixing skim plaster is where it all goes wrong for most people. The plaster must be mixed to a thick, creamy consistency which is a bit sloppy. There must 
be no lumps. The dry plaster must be added to water and mixed the water should not be added to the powder as with mixing mortar. If possible the mixing and movement should continue all the whle the powder is being added, as in the image. A special "paddle" blade is used, in conjunction with a variable speed electric drill, to get a good mix. The paddles are not expensive and can be bought from the tool store below or simply click on the image.
As you can see from the image left, plasterers use big buckets which enables them to mix a whole bag of skim in one go. It is unwise to do this if you have no experience as a bag of skim plaster covers about 12 - 13 square meters which an amateur cannot lay on in one go without it going hard. Clean cold water should be put in a 2 gallon bucket first to about one third of the depth. Then finishing plaster tipped in gently, until the heap rises above the surface of the water. The paddle, on a low speed, should then be placed in the bucket and moved up and down, and side to side, to mix the plaster. You may have to add more plaster to get a consistency to that of thick porridge.
Always have another bucket of water close, to clean off your tools immediately, including the paddle. If you mix a new batch of plaster and there is still some of the old remaining on any of the tools, it will go hard much quicker. The mixing bucket should be cleaned thoroughly after every mix.
Ceilings are the hardest to do so practice is vital. The series of images show what to do and how to do it but the "feel" of doing it can only be gained by 
practice. When mixed the plaster should be tipped out onto a board which is raised off the ground high enough to get a hawk underneath. Plaster should then be drawn onto the hawk. To start with only a small amount of plaster should be placed on the hawk. The hawk should be tilted towards you as you scoop some skim off the heap with your trowel. This is done in one swift, smooth movement. Plastering is not easy, even this takes practice.
Lay the plaster on the ceiling as quickly as you can and keep the thickness even. Do not worry about trowel marks just yet. Do not attempt to get the skim smooth at this stage. Your concentration should be on applying an even thickness (One coat = 3mm thick or preferably 2 coats @1.5mm) over the area as quickly as possible. If you are applying two coats, the second coat can be laid on 
top of the first immediately. You then go back to where you started to begin "trowelling up". Its now time to get the surface as smooth as you can and if 
the plaster has just started its "set" you should be able to remove the trowel marks. If not, a third pass will be necessary. Do not spend too long around pipes, architraves etc until the set has started, concentrate on the main areas but keep an eye on the set making sure you do not leave it too late to get a good finish with a "small tool" around the pipes etc. As you trowel up you will find the float naturally drags plaster from high spots and applys them to lower areas but you can add a little more plaster to very low areas and wipe any surplus from your float onto the hawk.A final pass can be made easy, if the plaster is setting fairly hard, by applying clean water with a wide brush and following the brush with your float. A lot of plasterers carry a rose spray gun to spray a fine water spray to the surface to get a great smooth finish when trowelling up.
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