Prepare your wall or ceiling

Skimming Plasterboard

If you are skimming a plasterboard wall then the suction test is not needed as the paper controls the suction for you.


prepare wall for plaster

Skimming onto a suitably scratched backing coat.

A suitably scratched backing coat means that it should be rough enough so that the surface area is increased and the plaster will have something to grip onto.

If you don't prepare your wall then no matter how good you are at skimming it could all go wrong!

Controlling the suction is crucial to obtaining good results. If the wall you are plastering is porous then the plaster will firm up too quickly (due to the moisture being sucked out of it) before you have had a chance to flatten it. It doesn't matter how good your technique is – if the suction is not controlled properly then it can be impossible to achieve good results.


Suction Test

To find out if the wall you are about to plaster is too dry

  • Load your trowel with a small amount of plaster and apply anywhere in the wall to ensure you have an area of plaster of approx 50mm x 50mm. The plaster should be approx 2mm thick.
  • Leave on the wall for 2-3 minutes.
  • Test plaster spot with fingers. If it is still as wet as when you put it on - then the wall is not high suction and will not need to be soaked. If however it has dried significantly in this period of time then you have a dry wall which needs soaking with more water.

When soaking with water care should be taken to apply it evenly to ensure even drying across the wall.


Skimming or plastering a previously plastered surface or painted surface

Prepare your wall - Adhesion

If you are skimming a previously plastered surface or one that has been painted then adhesion may be a problem. PVA should be applied if the substrate is low suction or if it is high suction and there is no physical key for the plaster to grip onto.

Low Suction

A background that provides no suction at all must be properly prepared or the plaster will not stick. The use of a bonding agent such as PVA will need to be painted to the wall to ensure adhesion. PVA should be diluted with water (follow instructions on can). A handful of sand mixed in with the PVA will provide a rougher texture when the PVA has dried – ensuring better adhesion.

1 coat of PVA will be sufficient.

High Suction but no physical key

The plaster will bond initially due to the substrate being porous - however if a physical key is not present then in time the plaster could lose it's bond and so even though it is porous it either needs to be scratched up a bit to provide a greater key or simply use PVA.

The water may be sucked out of the plaster straight away on a high suction surface. The more coats of PVA that are applied the more the surface will be sealed and the suction lowered.

Suction Test 1

As you are applying the PVA notice how it is drying. As it dries it will change from white to clear. If this happens within 15 minutes then apply another coat. If you're applying to a white wall and you can't see it - just feel it. If it is still wet or tacky after about 15 minutes then you should be okay. If it is dry after 15 minutes then you may still have high suction and another coat of PVA may be required. Some walls need just 1 coat however some may need 3.

3 thin coats are better than 1 thick coat - if the PVA is too thick it will just lay on the surface and risk peeling off. The thinner it is the more it will be absorbed into the wall and really adhere.


Suction Test 2

To find out if the wall you are about to plaster is too dry

  • Load your trowel with a small amount of plaster and apply anywhere in the wall to ensure you have an area of plaster of approx 50mm x 50mm. The plaster should be approx 2mm thick.
  • Leave on the wall for 2-3 minutes.
  • Test plaster spot with fingers. If it is still as wet as when you put it on - then the wall is not high suction and will not need to be soaked. If however it has dried significantly in this period of time then you have a dry wall which needs another coat of PVA.

PVA can be purchased at any builders merchants or DIY store. .

Beginners

When starting out your arm may not be use to all the physical work. A lot of pressure is required on the trowel in the later stages - so you can make it easier for yourself by applying the PVA the previous day.

Normally it is good to apply the plaster when the PVA is tacky - so if you're intending to let it dry, then put a handful of soft sand or finish plaster in with the PVA solution. Then when this solution has been applied and left to dry - a rough sandpaper like texture will result, to which the plaster will adhere to nicely.



Avoiding problems

Always ensure everything that you use for skimming (bucket, tools etc.) is totally clean. The finish is applied very thinly so even small bits of grit can cause big problems.

When tipping out mixed plaster on to a board – make sure the board is totally clean. If you are using the same board as you did for the backing coat then it may be a good idea to turn it over and use the other side. Ensure the board is soaked with plenty of water first to ensure the plaster doesn't dry out.

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The author cannot accept liability for the use of any of the materials or methods recommended in this manual or for any consequences arising out of their use. The author cannot guarantee the suitability of any recommendations made in this manual and shall not be under any legal liability of any kind in respect of or arising out of the content of this guide.