If you are skimming a plasterboard wall then the suction test is not needed as the paper controls the suction for you.
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
When soaking with water care should be taken to apply it evenly to ensure even drying across the wall.
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.
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.
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
PVA can be purchased at any builders merchants or DIY store. .
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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