Rendering on internal Sandstone wall

[sammy02]

Hi there

1st post here but from the reading this looks like a great site.

I've a small DIY job on, need to finish an uneven stone (party) wall in front room, approx 3 x 2.5m area. The wall is pretty dusty and bonded with original (1904) lime mortar. Finally, it is up to 1.5 - 2 inches out of plumb in places.

I'm planning was planning a 3 coat application:

1. Sand, lime and cement 5.1.1
2. Sand lime and cement, 6.1.1
3. Gypsum plaster skim.

Will the gypsum skim harden off too much, or too quickly causing blistering or cracking?

Also I've recently been advised by a professional restorer (national trust) that I should be scraping away all of the surface mortar and about 3/4 of an inch of the joints before applying the 1st coat? Won't a PVA seal or wetting down the wall achieve the same thing?

Any advice appreciated.


P.S. Injection DPC already installed, no damp problems on the wall, at all.

 

[Micilin]

Do you have to plumb it up?

 

[sammy02]

I don't have to but I want to, the first coat would really only fill the big holes in.

Thanks

 

[Micilin]

I imagine that the NT fella is advising you

- to get off all the loose stuff so that you are not going on to a bad background. any kind of plastering is only as good as what it goes on to.

- to rake out joints as he doesn't expect you to use PVA etc, but have a physical key This key may be helped by removing all the old coat.

If you still need a key, PVA on it's own is not ideal for S& C.

A better option would be a slurry of sand:cementVA ( I always SBR which is designed for cement, but if you are inside with no damp, I'm sure PVA is fine -others??)

This can be brushed on or thrown on to the dampened cleaned wall, and after a couple of days try getting it off!

Oridnary sand and cement will then do you to float, but build it up in scratch coats (slightly weaker each time) to give you a final float coat of even thickness.

You could skim this, or finish with sponge and paint depending on what you want.

The above is with the caveat that you are sure you are allowed to do it, bearing in mind you have a NT guy to ask and that a flat plumb wall is in keeping with the rest of the room/house

 

[sammy02]

micilin

Sorry, I missed the point there, The property isn't listed, in a conservation area or subject to anything but normal part p regs. The NT guy is an acquaintance and hasn't been round on their behalf.

Thanks very much for the comments, what ratios would you use for your slurry?