The main difference between plastering onto plasterboard and re plastering an existing plastered wall is in the preparation.
Existing dry plaster of any age has a high suction capability. It quickly draws the moisture from the new plaster being applied. This makes trowelling a difficult process as the time to work the plaster is dramatically reduces if the preparation is not completed properly.
Firstly make sure that the wall to be re plastered is free of loose areas of plaster that may fall off. These would need to be hacked back to brick work and repaired with bonding using the methods described to patch repair a wall with bonding plaster.
Damaged plasterboard walls should be repaired as described in the repairing plasterboard section of the notes.
The next process is to reduce the capacity of the old plaster to accelerate the rate at which the re skim plaster hardens. This is done by applying 1 or 2 coats of diluted PVA glue.
Most PVA glue carries instruction of the dilutions required for a range of different jobs. Preparing a wall for re plastering requires a dilution of around 3 parts water to 1 part PVA glue.
The diluted PVA glue should be generously applied to the wall using a heavy brush or paint roller. No part of the wall should be missed likewise the wall should be covered evenly to produce a predictably constant setting rate for the plaster across the wall.
PVA glue can be applied immediately before the skim coat is applied having allowed the PVA glue dilution to have become tacky before plastering. Alternatively an extra coat can be applied and both allowed to go dry in advance of the re plastering of the wall. This is a guide only and the PVA manufacturers’ instructions should be followed.
Never over PVA a wall as this will cause the plaster to blisters and not take to the wall resulting in it peeling or falling off in days or years to come, especially during re decorating.
Always run a dilution of PVA along the edge of walls adjacent to the wall that is to be re plastered.
With regards to skirting boards and other fixings that are in place when re plastering. Cut or scrape back a narrow channel against the fixing into which the new plaster can be applied. The scraping back should occur before the PVA process.
If re plastering a wall with external skim corner beads in place, scrape these back. You can apply new beads but this will require that the returning wall is also re skimmed. If that is the intention then we suggest that new beading is applied over the old beading using plasterboard adhesive or bonding plaster. See how to apply beads in the notes.
Now follow the process of how to skim plasterboard.