How to skimming coat process

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The Finish Coat

The skim (finish) coat is the final coat and is responsible for smoothness. It is not designed to flatten so the background to which it is being applied must be flat – if not it can be near impossible to remove trowel marks.

Not only this but an uneven background can result in some areas having a greater thickness of plaster. The firming up process is influenced by the thickness of plaster. When the plaster begins to firm up - it behaves differently so whilst cannot be flattened so easily - holes and hollow areas can be filled and trowel marks removed.

Therefore if one part of the wall is twice as thick as another, it could take twice as long for it to firm up, which means it cannot be finished like the areas that are becoming firm. This is inconvenient if confined to one edge of the wall but if there are several areas like this then it can result in an inconsistent and sub standard finish.

The importance of a flat backing coat can't be underestimated.

An understanding of all the stages of skimming is essential in achieving a nice flat and smooth finish. It is not just a case of applying the plaster and immediately smoothing it out: if you feel you are going round in circles, solving one problem but creating another, there's a chance it is too wet. Walking away from the wall and allowing it firm up a bit before you try and complete the next stage may be all that is needed!

There are 6 stages of skimming to complete - once you know what you are aiming for at each different stage, and how to carry this objective out, skimming becomes much easier - honest!

Materials for Skimming

There are many finish plasters available and they all have different characteristics. One of the easiest finish plasters for a beginner to use is – 'Thistle Multi Finish'. Being used my most professionals it is widely available and can be used on any surface.

The plaster should be mixed in a bucket with a whisk attached to a drill. Always put the water (clean, cold tap water) in the bucket first. If you want a full bucket full of plaster then the bucket will need to be half filled with water. By the time the plaster is added it will almost have doubled in volume.

'Multi Finish will set within 1½ hours - but remains workable for application for around 3/4 hour. You should aim to have applied the plaster during this time. It is always advisable to clean the bucket as soon as it has been emptied.

Old plaster or dirty water can bring the setting time down to as little as 5 minutes. Any unused plaster in the bag should be properly sealed and if not going to be used within a few weeks - placed in a secondary plastic bag to keep the moisture out.

Clean water must be used for mixing - especially not water that may have old plaster in it from cleaning tools previously. If during the skimming process another batch of plaster needs to be mixed then it should be kept separate from a previous mix – as they will be setting at different times.

If you are skimming on to plasterboard then all joints should first be taped up with plasterer’s jointing tape (sometimes referred to as 'scrim'). This will stop the joints cracking. The tape is self adhesive. If you have any big gaps then sometimes two layers of tape may be beneficial.

Tools

No matter how good your technique is - if your trowel is not prepared properly them it can be impossible to be successful with your plastering finish.

Using Your Trowel

It is important to have a trowel that is suitable for the job.

A brand new trowel out the box is fine for applying the backing coat but when it comes to the finish skim coat it is important to 'wear it in'. Otherwise it will be impossible to get a nice finish and the sharp points on the corners will forever be digging in to your wall. Plus even more importantly the square edges will almost cut into the plaster and will be far too harsh.

It is possible to buy 'pre-worn in' trowels but sometimes they still need a bit more work - it literally just takes a couple of minutes.

The corners need rounding off with a file or on a smooth concrete floor / paving. Once all four corners are slightly rounded all edges of the trowel can be gently smoothed off, again on a smooth concrete floor. But care should be taken to ensure the whole edge is smoothed and rounded in its entirety.

Whilst this is taking place putting a reasonable amount of pressure on to the trowel will encourage the blade to bend very slightly, meaning you will have a little tolerance if the angle of your trowel is not correct when applying the plaster.

Now this has been carried out just check that there are no harsh square edges - every edge should be slightly rounded and smooth.

Size of your trowel

Smaller can be better!

To achieve a good finish - it is important to push hard on the trowel. This not only keeps the plaster flat but also forces the plaster along the whole length of your trowel and into any hollow areas - hopefully to leave a nice even flat surface free of holes.

Therefore as a beginner a mall trowel is not only easier to control the angle (how far the leading edge is away from the wall) but also means more pressure can be applied over the smaller surface area.

The pressure you are able to apply as a beginner can vary due to your arm aching - therefore maximum pressure on a smaller surface area of the trowel can only be good (as long as you have the correct trowel technique)

Local Plastering provides great services while still remaining affordable to fit into your budget.
We have worked with a wide range of customers in houses of various sizes. We have the knowledge and man power  to work with many construction projects simultaneously, allowing your project to be concluded on time.