Pre worn trowels are trowels where the edges are already at a taper, you can by pre worn trowels but there are not fully worn down just a start for them to be on there way, for regular plasterers its wise to have one trowel that's worn for finish coats and a not so worn one for undercoats, so by the time your worn one has had it you have another worn one ready to use for finish coats, then you buy another pre worn for undercoats so that one can get the fully worn state. This way you always have a fully worn trowel.
When you buy a Pre worn Trowel you can grinde the edges on a oil stone to get an angle more like a fully worn trowel.
Plastering - How To Skim Coat
If you're after a smooth flat wall then you will need to skim coat
A skim coat is basically 2 very thin layers of finishing plaster applied on top of each other then smoothed and flattened down to create the finish your after, getting this finish is where a lot of skill and practice comes into play.
Using the right tools is also essential and this guide will talk and show you the process from start to finish including how to use the tools and the techniques involved. Whether you're plastering onto render, undercoat, plasterboard or over an old skim coat these are the methods you should follow to getting the perfect finish.
Prepping
If you are skim coating directly on to plasterboard then you can skip this step.
First you have to make sure that all loose material is off the wall, you can do this with a hand dust pan brush, brushing the wall from top downwards, if there is large loose bits then you will need to follow the repairing plaster section here.
In order for there to be a key for the plaster to grip to there needs to be no dust or and little loose material at all, as if there is, the bond will be weak. We get this from a liquid glue commonly known as PVA, its best not to use your kids though and buy PVA that's designed for this. You will find it's marked on the container that it can be used while plastering, they commonly come in 5 litre tubs.
PVA
You need to apply the PVA to the wall but first you need to water it down, so get a paint kettle or bucket or other container and mix some PVA with water (5 parts Water to every 1 part PVA) and mix till the PVA is dissolved. Sometimes a mix of 3-1 is required on high suction backgrounds.
To apply your mix to the wall you need a masonry brush, Starting at the top working your way down the wall, the reason you start at the top is the PVA will drip and run down the wall where your just about to apply more and brush over, this ensures a nice even coat.
Apply the PVA generously, 1/2 a pint per square meter should be enough but on materials with high suction this can be more like 3/4 of a pint per square meter and more then one coat.
This step ensures that there is a nice bond of the plaster to the wall your plastering, ensuring a nice strong grip.
Its best to wait till the PVA is tacky before you apply the plaster but if you wait till the PVA is dry and its been no longer then 24 hours its perfectly fine as the moisture in the plaster will re-activate the PVA.
First Skim Coat
The first coat is more about getting the wall as flat as you can, you should aim at getting a thickness of around 2mm.
Mixing
The idea here is to try and mix enough to do one wall, you do one wall at a time so you have time in between to flatten out your work.
Get a buck large enough for a mix on one wall, pour in enough water then start to stir while adding the plaster. The reason you put the water in first is to ensure the mix is even, non lumpy and mixes faster.
The amount of water to pour in first does depend a little, as a rule a quarter or a fifth of the bucket, you can always add more water when the mix is on its way and even more plaster if need be.
Mix till the consistency is around thick melted ice cream, if you place a stick upright into the centre of the bucket it should only just stay up when the plaster is ready. See Fig 1.
The tool to use while mixing is a paddle and drill.
The Hawk
Probably the most daunting tool as well as the process of getting the plaster from the hawk to the trowel.
Getting the plaster on the hawk is the easy bit, you hold the hawk up and using the bucket trowel of other tool of choose you scoop the plaster up and place it on to the hawk. Don't over load it, about 3 generous scoops is enough.
Getting the plaster from the hawk to the trowel is the tricky bit, the key to this is all in the movement of the hawk and remembering the plaster is designed to not drip down the wall so its not going to fall off the hawk while at an angle.
You hold the trowel so the base is facing up, you then tilt the hawk 
towards the trowel until at 45° and start scooping up taking only a thin layer off at a time. Never tilt the trowel and always hold the hawk upright when not scooping and no flicking of the wrist as this can cause the plaster to fly off.
Troweling
This is the part someone that's attempting this for the first time is going to get some on the floor.
Always keeping the trowel base facing upwards you start at the top left of the wall, place the trowel to the wall still upright, at this stage if you like you can do the same with the hawk under the trowel to catch any plaster that may fall.
Now you tilt the trowel until the plaster makes contact (around 30°) then spread down then back up again, making as long strokes as you can. While away from the ceiling you can trowel up then down as this makes it a little easier.
Continue this across the wall until the wall is fully covered, brushing out your lines as you go with your 4" soft brush.
Flattening
After waiting 10 mins you get your 4" brush, some water and the feather edge ready.
First place the feather edge flat to the wall and place your level on the feather edge, this will tell you if the wall is not level, from here you can work out where you need to remove some material to make if flat.
Feather Edge
You tend to use a feather edge across the wall rather then down the wall, but if you feel you would benefit from it then do it upwards.
The was you use the feather edge is to only take of a little bit at a time, you always have the thin side to the wall with the angled wedge shape facing away from the direction your scraping. The angle is there so you can get into corners better.
You throw a bit of water on to the feather edge and wall then scrape in a zig zag motion, keeping the feather edge as smooth and level as possible, taking as long scrapes as you can, once you finish a scrape wipe down the feather edge to get rid of any plaster you scraped off then you can carry on scraping where you haven't already.
Fill any low spots with a little bit of plaster and your trowel and scrape with feather edge again until its as flat, level and uniform as you can get it.
The more used to plastering you get the less likely you will need to use a feather edge.
That's it - now follow the recommended dry time that's written on the bag of plaster before you continue to the next coat.
Second Final Coat
Not to much different from the first coat although your only putting on a layer around 1mm and this is where even more trowel control is needed.
Troweling
You can put even less on your trowel now, starting from the top left working to the bottom right.
Now to get the best finish you need a trowel that's been worn in, a worn in trowel is one that's been well used and the edges are shaped as a wedge (tapered), this taper helps get the smooth finish and is less likely to leave trowel lines behind.
The idea now is to not even leave faint lines made by the trowel, what you need to do is trowel 
out the lines made by your trailing edge with your leading edge.
Doing this will take some practice, The trailing edge is the edge you use to spread the plaster with and the leading edge is the opposite edge. The trick is the angles used, you spread up with an angle of around 6° from the wall then down again at 3°, so as you spread up you may leave some lines, so on the down stroke as its a tighter angle there is less of the edge to make lines with and it evens out the plaster more so the lines already made disappear.
Be very careful though as if your trowel base touches the plaster while flat 0° its going to me sucked in and rain the finish, if this happens the worse you can do is just rip the trowel off the wall, you want to very slowly move the trowel over the surface slightly turning it to an angle and it should come again without to much damage, you can just re-trowel this again.
Smoothing Out
This is what gives you the final finish, again you want to wait 10 minutes before you do this and if this coat need flattening follow the feather edge section again before smoothing out.
Smoothing out is basically like troweling but without having plaster on your trowel.
You throw water on to the wall by flicking it off your 4" soft brush, dipping your trowel into a bucket of water you then glide the trowel lightly and a very tight angle across the surface of the plaster, again long strokes.
Keep throwing water in the area your just about smooth over and clean your trowel every now and again.
Corners
Internal
As you do one wall a time you shouldn't have to much problems with internal corners as while one wall has started to harden its easy to get the trowel up to the wall without damaging the finishing, if there corner is still a little out you can use a brush to even things out or 
even a internal corner trowel.
External
A little harder but the use of corner beading helps.
To fix this bead you can Nail, Screw or Glue it in place, i wouldn't recommend nailing or screwing if you have a brick wall as doing the so close to the edge could blow part of the brick out.
The idea of the corner bead is to bring more strength to the corner and to provide a edge to trowel on.
on your first coat you can use the corner bead as a depth guide, so sliding the trowel over the bead while troweling will get it perfect.
On your second coat you need to make sure the metal isn't showing so you need to glide 1mm over the bead, once one wall is curing the jointing wall can use your over hang to plaster to and use as the guide.
If you build a corner up to much you can always sand to a nice finish once cured, but you could also brush the corner after 10 mins and even use an external corner trowel.
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