Renovating old domestic plaster surfaces

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applying the first coat of finishing plaster

A testing job for my own recipe JBLime finishing plaster but one that any other lime plaster may well have not managed so easily.

The walls are generally old lime plaster but have been painted with emulsion, wall papered and patched and in places over skimmed with nice pink gypsum. The main patches were done with limelite.

In the picture you can see the new finishing plaster going on (the darker grey)  Areas where limelite was removed and replaced with a new lime plaster and the green is the wall prepared with a grit primer after the wall paper was removed.

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The fire place was a bit of fun. The three walls are gypsum apart from the one on the left which is half plastered in cement. They were all painted with emulsion which was easily scrape off. I keyed the gypsum to get a good mechanical fix for the plaster skim.

The wall can be seen with the grip primer, I did not want to entirely rely on the grip coat so I also keyed the old plaster with my spiked roller (lime plastering plasterboard. with the correct tool and material it works well.)  which has come into its own for breaking through the surface of painted plaster. It takes a fair bit of force but it soon does the job and gives a decent key without making a great lot of damage, it is also useful for stressing the plaster all over so that any loose or suspect bits of plaster are soon found out and can be replaced.

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The final coat applied, all smoothed and flattened. It is still drying here and there; on a job like this, with all the different substrates to contend with, drying is very uneven. The new lime patches at the bottom reached trowelling condition within a couple of hours, the plaster on the cement took two days. The plaster on the gypsum was reasonably quick being trowelled the following day, which just called for patience because as I said, the left hand side was gypsum down to half height then cement, which, to add a little extra difficulty, was set back from the gypsum half about 3mm so it needed and extra coat of plaster. The rest of the grit primed plaster reached trowelling condition about 24 hrs after application.

When I say trowelling I mean flattening, smoothing and compacting.  The window of opportunity for trowelling is quite wide, although there is a sweet point when it is perfect. If you try too early the plaster can form the occasional little bubble and it is very difficult to remove the trowel marks, trowelling too soon is best avoided but there are times when you need to do it, this is when I know that if I leave the trowelling overnight it will go too far and be almost too dry for the trowelling to be easy the following morning, in that situation I can carefully trowel the plaster smooth and then in the morning all I need do is go over again for the final compacting and smoothing.

Of course all this is so much easier when the plaster is flat, undulating plaster is such a chore and for that I find I have to use the really flexible, venetian plastering type trowel, but for all other plastering the trowels I use are rigid, that appears to be the Japanese tradition, rigid trowels for laying on, with practice you can get the plaster pretty flat without the need for floating, and even on this job, which was mainly skimming I used a very rigid Japanese Jigane trowel.  ( http://japaneseplastering.blogspot.co.uk/p/blog-page_10.html)

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Honyaki finishing trowel – the blade is quite rigid with only the slightest amount of flexing under pressure. It compacts and flattens to achieve a very hard wearing lime plaster surface.

 

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About byrnesurfaces

conserver and repairer of historic surfaces
This entry was posted in Lime Plaster, technical stuff. Bookmark the permalink.

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