Well, I am not a plasterer but I know how to plaster so I thought I might help the world and give my experiences with plastering so that you can have a go.
There are various plasters available and it depends on what you are plastering as to which sort you need. Basically there are undercoat plasters and there are finish plasters. All plasters are classified by the background they are designed to be applied to, some are for backgrounds that suck water a lot and others are for backgrounds that do not. So here is my advice:
1) One coat plaster is only really worthwhile for small areas, try and use separate plasters otherwise.
2) If you choose a plaster designed for low suction for a high suction background, it will crack as it dries. If it is a finish then you will have to start again or do loads of filling.
3) If you choose a plaster designed for high suction for a low suction background, it will take a long time to dry and this will affect the quality of the finish as you trowel it but is generally passable.
4) Make sure you get all the equipment you need ready to go before you start mixing anything. You will ideally need a bucket to mix in - a large one unless you are only plastering a small area (1 bag of finish will do about 10 square metres of wall). You will also need a finishing trowel, a spot board (a smooth board used as a table for the plaster), a hawk and something to get the plaster out of the bucket with. You will need something to mix the plaster with and the most popular are metal mixing paddles that fit into electric drills. You can use a battery drill but ideally it should be at least 18v because a lot of torque is required to stir wet plaster.
5) Finish can only be applied directly to walls that are basically flat and have no areas that suck a lot. If this is not the case, mix up an appropriate undercoat plaster so that it is a creamy consistency slightly tigher than a McDonalds milkshake. It should be able to make shapes in the bucket but it should have a wet look to it.
6) Don't take too long stirring it because it will start to harden but make sure there are no dry bits in it to cause lumps. You might have to use a scraper or something until you are comfortable using the paddle effectively.
7) Empty the bucket of plaster onto the spot board so it is quick and easy to get to. Use a bucket trowel or something similar to remove all the loose plaster from the mixing bucket and if you have an assitant, get them to clean the bucket and mixing paddle in cold water. Don't wash plaster down the sink!!
8) Scrape some of the plaster onto the hawk with the finishing trowel and use the finishing trowel to scrape plaster onto the wall. If you are undercoating, you might need about 10mm of plaster but if the wall is not flat, some areas might be more like 30 or 40mm deep but undercoat plasters can handle this.
9) Make sure that the plaster level is basically flat, you might need to buy a plasterers feather or use a dead flat piece of metal to run across the surface and make sure there are no major bumps or troughs. The flatter you get the undercoat plaster, the easier it will be to finish. Do NOT try and get a finish quality surface, you don't want it to be flat and it will waste time, just get the plaster on and make sure it is generally flat. Put any unused plaster into a bin bag or empty plaster bag and clean your spot board and trowel. Clean tools are happy tools!
10) Once the undercoat plaster starts to get firmer, scratch the surface in a diamond pattern every 4 or 5 inches apart to provide a key for the finish to grip onto.
11) Leave it to dry!
12) Finishing is a different beast, the tolerances are much closer, the plaster is applied more thinly and everything that goes wrong affects the look and might ruin the finish. You can always scratch it and do it again but you don't really want to.
13) The finish is mixed basically like the undercoat plaster to a creamy finish, do not make it too tight otherwise you will not be able to trowel it on.
14) It is very important that all lumps are mixed out of the finish, otherwise you will get holes in your finish that need to be fixed or filled later on.
15) Empty the plaster onto the spot board and wash the bucket and paddle.
16) The finish is applied in two stages, the first needs to be quick and will not be perfect - do not try and make it perfect. Stage one is simply covering the wall with a consistent surface of finish plaster only a few millimetres thick. If you have no lumps you should be able to do this fairly easily. If you get any major holes while you are skimming at this stage then put plaster over the top but you must do this now or wait until the plaster starts going off or you will pull the plaster from the wall.
17) You can trowel over any major marks AS YOU GO but do not be tempted to go back over previous plaster to correct things until stage 2. The end of stage 1 is to have a fairly flat but not perfect layer of plaster over the whole wall. When you are first learning to plaster, you will be slow which is why you should start with a small wall and then work up to a larger one.
18) Wait until the plaster starts to go off, that means if you put your finger onto it and pull away, it will not cause a little peak but will leave a finger mark. This means you are ready to start the trowelling and repairing.
19) At this point your wall will have various levels of defects so work in stages starting with the major defects and work down to the smaller ones. Using a wet trowel, flatten out any lines or peaks and if you have holes, take some of your existing plaster (do NOT mix up more plaster) and carefully push this into the holes as you trowel them. The more you do, the better your wall will become and you either stop when you are happy with the finish or when the plaster has dried so much that it will not trowel any more. Make sure you keep your trowel wet or it will pull plaster off of the wall. It is worth investing in a nice trowel for £30ish which will make trowelling much easier to do, especially when you are learning.
20) Any defects after the wall has dried can be sanded or filled with fine fill but unless you have plastered a lot, do not expect perfection here.
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