Patch repair of lime plaster so it blends in...

[Wren]

Hi

We have new window opening. Wall is lime plastered and limewashed. Would like to keep existing lime plaster and put in new lime plaster to marry in with it.

My other half has lime plastered a wall downstairs (over wooden laths) and did a good job of it, but it was a whole wall all on its own. When he has tried plastering a small section up to existing plaster finds it hard to get a seamless join.
In our previous house I have made small repairs to 'normal' plaster with filler, scraper, bit of sanding - but doing this with lime plaster is a different ball game.

If anyone has any advice please on how to patch repair lime plaster and blend it in nicely would be very grateful!
The lime plaster in this room was previously covered in horrible plastic-ey gray paint (meant to be some sort of anti-damp thing not sure) which I spent many days carefully scraping off, so am keen to return it to its original glory.

many thanks

 

[Wren]

I have found this on the Ty-Mawr website. The pictures are of a much smaller repair. Doesn't seem to suggest doing anything particularly special. So maybe it is just a case of choosing a small hole to repair working very slowly and carefully, and then build up practice/confidence to tackle the bigger repair jobs - where having to build several coats up from the stone wall all the way up final topcoat which meets perfectly with the existing plaster.

https://www.lime.org.uk/community/lime-plasterrender/repairing-interior-finishes.html

 

[Abi Cott]

Easyfil is quite nice to work with and seems to bond well to lime plaster. I tried some after seeing a couple of other members on here recommend it, and I've had some success with cable channels and the like. I have also worked in gypsum before, but I'm a novice with lime.

 

[Tom-H]

I’ve also had success using easy fill on the transition between the new and old lime plaster when repairing a similar sized patch. It will require filling and sanding a couple of times but will be perfect then.

You just need to be careful not to sand to deep as the smooth skin of lime plaster is quite thin.

 

[Flyfisher]

I’ve had some success using this fine lime filler to smooth out any hairline cracks and level differences:
http://www.anglialime.com/products/wet-readymixes-mortars-renders-plasters/superfine-filler

 

[Wren]

Thank you for all of the thoughts and tips. We need all the help we can get!
After searching around some more, I have found this bit of text on the PPUK website (credited to Old House Store, and Somerset Council):
".....cut back the edges...The best tool for this is a craft knife. ...Ensure that the edges of the patch are cut square to provide a key for the new mortar, and eliminate feather edging.
...apply the render in thin coats ...taking care to ensure that it is well pushed in beneath the undercut edges of existing plaster. Apply the final coat with a neat butt joint between the old and new
and with a slight bulge in the middle of the patch so that the final consolidation will create a flat patch, flush with the surrounding render.
Do not feather the edge of the patch over the surrounding plaster. Push back and consolidate the patch with a wood float."

So, carefully undercutting the existing plaster (with craft knife) sounds quite an important step.
Trying to do feather edging is probably where we have gone wrong before...
Think I get their description of creating a bulge which then need to flatten out, sounds like quite a skill though.

While we get round to that am going to get hold of some of that "fine lime filler" and start using it for some little repair jobs. The test will be putting coloured limewash on top, and seeing how that behaves.

Moving slowly towards the day we have a house that doesn't look like a building site.

 

[88v8]

Inevitably there will be a difference in absorbency between the new and old plaster. This will show up in early coats of limewash.
You know, incidentally, that one has to leave new plaster at least 3 months before applying limewash or clay paint. Three months after it's thoroughly dry.............. At least.
Here, one is referring to full-depth work where the wall was thoroughly wetted and the plaster has been repeatedly misted, there is no particular need for delay where small repairs are involved, although any application of lime plaster products requires a wetted surface.

Because it is thicker, clay paint may be more forgiving of absorbency differentials. Although, paradoxically, it is easier applied thinned, there have been some recent threads.

The greatest test of plastering is matching the texture. Slant lighting can be a severe test. Wall lights, oooer.
If there is residual wonkiness, embrace it.

But bear in mind if the end result is not as you envisaged, that the development of flat plaster came about to facilitate the application of.... wallpaper.

Ivor

lwog

 

[MatthewC]

I had replastering done on parts of walls in several rooms, not wanting to strip good original plaster but having huge issues caused by water ingress in places (see my blog for late 2010, click the contact blob on the right of this). I then avoided this problem entirely, as I thought that the smooth joint would be impossible! After replastering base and top coats, we then painted DG27 on the old stuff and gave the whole wall a single coat of fine plaster (Mike Wye "Regency" it was at the time). Worked a treat and still looks good.

".....cut back the edges...The best tool for this is a craft knife. ...Ensure that the edges of the patch are cut square to provide a key for the new mortar, and eliminate feather edging.
...apply the render in thin coats ...taking care to ensure that it is well pushed in beneath the undercut edges of existing plaster. Apply the final coat with a neat butt joint between the old and new
...
So, carefully undercutting the existing plaster (with craft knife) sounds quite an important step...

For cutting the edges, I found that a decent long bladed Stanley knife (the sort with blades that you can break bits off) was the best tool for this, with a 50-75mm blade. Two points to note: what you need to do is to avoid disturbing the plaster you want to keep, so (IMHO) you need a ALMOST square cut back, but I didn't do a perfectly square one, and certainly NOT an undercut. I found that a very slight angle was less likely to damage the plaster that you want to keep, so I was cutting at 75 deg or so, not 90 deg or more. The second point is to make your initial trim back about 75mm from where you think you want to be, and then do a second cut for that last bit. This is because when you cut a large chunk the cut itself or the weight of the plaster is likely to affect the remaining plaster and loosen it; when you cut only 75mm the second time, it will break more easily and so not damage the remaining plaster. Also, at no stage should your blade be directed close towards plaster being kept, for the same reason. I hope that's all clear!

Matthew