I just recently bought an 1850's stone farmhouse. The interior walls have some repair work that is needed and I can't find anything that addresses my exact situation.
Here is an example of a patch that I want to fix. Some are bigger than this.
There is an old lime/clay/mud mortar used between the stones. Its is VERY crumbly. On top of that is old horse hair plaster. It appears as if it's one solid coat. The plaster is actually still pretty hard - it's the old mortar that seems to be leading to the plaster cracking and falling off.
In the photo, working from top down, is a stone, followed by a big piece of the old mortar, followed by a stone, a small gap and another stone. The discoloration is just some wetness, i tried to wet the stone to get a better picture
I'd like to patch these areas but just haven't had any luck.
First I used some gypsolite plaster as a basecoat because some of the holes are pretty deep. That didn't work very well because no bond was created. It pulled off of the old mortar very easily. It also pulled off of the old plaster easily. Not sure if the gypsolite is incompatible with the old horsehair - It definitely didn't mesh well with the old mortar.
Then I tried using Larsens Plaster Weld. I painted it over everything, old plaster & old mortar. The gypsolite bonded better to the old plaster but it still pulled away from the old mortar. I also ended up with a lot of cracks. I figure this is due to the old dry mortar sucking out the plaster moisture too quickly.
Has anyone out there had any experience with a repair like this?? Most of what I find is plaster/lath repair - occasionally plaster over brick but that's about it.