Trowelling plaster' means to 'work' plaster that has already been applied.
Aim – fill in all holes. The plaster is pliable enough to push into holes but not too wet so as to create them. When the next stage is carried out the plaster will be very firm and it will be difficult too fill any holes – so ensure all are filled at this stage.
When – the plaster should be tacky – if it is too wet the plaster will be easily pushed out of shape and so creating more holes.
How – Firm pressure will push plaster along the whole length of your trowel and therefore be pushed into the holes. You may notice ridges caused by the trowel get worse when doing this but that is not a problem as these will be pushed out in the next stage when the plaster is a little firmer.
If the suction was not controlled, or the application took a bit longer and the plaster has firmed up and will not with much pressure push flat then water should be flicked onto the wall with a brush or it can be applied with a mister.
1. Pressure and optimum angle
You cannot push too hard when trowelling plaster – as long as you have the correct angle of your trowel. Find the correct angle and no matter how many times you run your trowel over the plaster it will not scrape off any plaster but just act to push the highs into the lows and flatten - ensuring that the whole length of the trowel is in contact with the entire surface, If you have the leading edge to far away from the surface then you risk scraping. Most trowels require the leading edge to be 10 - 15mm away, however some trowels that tend to bend may need more.
FeelingWhen you get the correct angle the trowel glides over the wall. If it is too flat it can drag - if it is too far away then it can scrape
You should be able to run your trowel over the wall and push the plaster around to flatten. If when you run your trowel over the wall it starts to gather plaster then all you are doing is scraping the second coat off.
To fill the hollows you want to push plaster in to them not scrape of all the surrounding area to the level of the hollow. However if you are pushing as hard as you can and the plaster is not being forced across the whole length of your trowel then you have two options:
If you have been over an area 3 times and the hole is still visible then more plaster should be applied in to hole and then flattened with your trowel stroke. Your trowel strokes should always be long and follow the format you used to apply the plaster - to avoid making lots of trowel marks everywhere.
3. Scraping & Filling
there are many shallow holes - the highs can be scraped off - the
plaster will collect on your trowel to then in another trowel stroke be
pushed into the lows. This technique is not ideal but if the plaster has
started to firm up and it is impossible to apply the required pressure
to push the plaster flat - then it can be a very valuable technique.
As your trowel passes over each area of the walls it should be studied for holes. Be thorough and do not move on until they have been rectified. If you are concerned the plaster is getting too firm then move quickly across the wall ensuring all large holes are filled. Then repeat this stage again for the smaller holes. If when you are doing this you realise there are a lot of holes then sometimes it can be quicker to go back to stage 3 and put another coat over the wall.
The corners can also be formed by scraping off the plaster from the wall you are skimming up to simply by scraping off with the tip of trowel. Do ensure all plaster is removed right into the corner. Try not to cut into the skim you have just applied on the main wall. Once the corner is free from excess plaster the corner can be formed with a brush. A paint brush (approx 75mm) will do the job. Wet the brush then flick off excess water. Ensuring the brush is pointed more at the wall you are skimming up to, run the brush in big strokes up and down the corner. Generally at this stage the plaster can be quite wet so little pressure is required. Any brush marks left in the corner can be lightly trowelled out – but again if it is really wet it may be better to leave until the next stage.
Aim – Remove all ridges caused by the trowel
When – if with firm pressure you can successfully flatten the ridges without creating any then it is ready. If you solve one problem and create another then it is too wet. You may be surprised just how firm the plaster has to be.
How – firm pressure. The leading edge of your trowel will be 10 – 15mm away from the wall. Combine this with firm pressure and you should succeed in flattening the wall and removing any remaining trowel marks. If any areas of the plaster have firmed up a lot and you can't remove any blemishes or ridges then bringing the leading edge of your trowel away from the wall slightly should help you. (Make sure you don't scrape all the plaster off though). For very small problem areas just using the front edge of your trowel can flatten off any stubborn areas.
The corners can also be formed in exactly the same manner as described in stage 4. (plenty of brushing will ensure any small holes in the corner get filled in.) Using a combination of your brush and trowel in the corner should leave you with a nice clean and straight corner.
Trowelling Plaster - final finish
Aim – achieve an even slightly polished surface. Some areas that may not have dried as quick may not be as flat and smooth as the rest due to not firming up enough. In this final stage of trowelling plaster all the plaster is firming up at the same rate due to the setting process, rather than the drying process allowing an even finish to be obtained.
When – as the plaster sets it will start to darken in colour indicating it is time for the final trowel.
How– Firm even pressure over the entire wall, concentrating on any individual areas if need be. As the plaster will be hard the trowel angle is not so crucial – affording a slightly scraping effect to be used on any blemishes.
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