Applying the backing coat to a patch
Repairing Plastered Walls
If existing plaster needs to be removed due to the plaster actually being loose then it will be necessary to remove it. Care should be taken when removing plaster to ensure that 'good' plaster is not loosened in the process. Always avoid getting your trowel or other tool that you are using behind the plaster and levering it away from the wall as this will loosen plaster that may have been ok. A chopping effect directly in to the plaster will ensure that not too much damage to sound plaster occurs.
Sometimes old plaster (particularly lime mortar) can sound hollow when tapped – but does not actual move when pushed. As a general guide if any plaster is loose then it is best to be removed – but if it just sounds hollow and there is no movement then it may be possible to leave well alone.
Once all the loose plaster has been removed - the most important thing to be aware of when applying the new plaster is to prepare the join properly.
The old plaster that you are joining up to may be very dry and possibly porous. It will therefore be necessary to wet this edge will liberal amounts of water or a weak solution of PVA to control the suction.
If the suction is not controlled properly - the moisture will be sucked out of the new plaster very fast and the plaster may shrink away from the existing resulting in not only a crack but the two will never be properly joined – possibly causing problems in the future. The area of the wall to be plastered could also be dry and porous – requiring soaking with water.
1. Prepare the join and control suction on existing plaster and wall .
2. Apply thin 1st coat of backing coat plaster ensuring plaster is properly pushed onto the wall – paying particular attention to the join to ensure plaster is pushed firmly into place.
3. Apply 2nd coat of plaster the correct thickness. Not so much pressure will be required.
4. Rule off the excess plaster with a straight edge. The straight edge should be long enough to reach from either edge of existing plaster to ensure the area will be in line with the existing wall. If you have no existing plaster to work to then it may be necessary to set a guide up. These guides can be adapted to suit any wall – sometimes it may be necessary to have a vertical guide rather than a horizontal guide.
5. Any hollow areas can be filled and ruled off in the normal manner (pg 13). The edge of existing plaster should always be kept clean by scraping any plaster off with your trowel – otherwise the straight edge will not get back to the true level of your wall.
6. Floating should pay special attention the join. If the float is always kept inside the patch and used to gradually work up to the join – this will have the effect of slightly recessing the backing coat so as to allow room for the finish coat.
7. Before the plaster sets it should be scraped off the existing plastered surface. The trowel should then be taken over the patched area and carefully scrape away any areas that are not recessed adequately near to the join so as to prepare the patch for the finish coat
Next Page - The Skimming Process
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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.