Plaster Clarification

[Geoffrey]

Another thing I was pondering...

Whilst having a quick lurk here a week or so ago I saw a thread that touched on gypsum and it's porocity. I was wondering (my house having a mix of gypsum, lime plaster and gypsum skims over old lime) if when sorting out various damp problems I need to do any replastering - If I use a gypsum plaster in areas where that is what's already there, and then use a breathable baint on top, will it stop the plaster getting shot if there is still a bit of damp.
I understand that it's not really wise to patch gypsum with lime or vice versa but don't want to have to replaster vast expanses in lime that have always been gypsum, just for the sake of a foot or so at ground level that suffers from the damp.
We've been here about 18 months now and hopefully the damp is being cured by the repairs and alterations we have made to drains, drainage and ground levels etc. But given the obvious evidence that damp has been a problem in the past, and that there is still moisture penetrating the stone floor (see other post), I need to plan the repairs and decoration to internal walls in full expectation that there will still be some damp penetration from below.

Sorry if any of this seems muddled - trying not to chase my tail too much!

Geoff

 

[Nemesis]

Well - we have lime patched with gypsum in places, but it's dry (on a staircase) and it's coming off very soon to be repatched with lime.

I think if it's not totally saturated a bit of gypsum may be OK, as a temporary repair to gypsum... but in the longer term of course seek to replace it.

Maybe others may have other ideas about the gypsum...

 

[NT]

Gypsum itself is breathable, though the paint on it may not be. It doesnt accomodate movement the way lime does and doesnt survive damp as well. For diy use lime has the advantage of long open time, whereas fast setting gypsum demands fast working or you get a mess of a finish.


NT

 

[Geoffrey]

Thanks Both,

So if I were to replaster just the bottom 2 or 3 feet or even just patch with lime would that or work? Or would I have problems where the 2 different plasters meet? I know I cant't easily limewash onto gypsum but that's not a problem as i intend to use breathable chalk or clay paints and glazes instead.

 

[monkey writer]

i'd be interested to kno0w this too...

 

[BMcD]

When Gypsum finish is applied over the top of lime plaster it is akin to icing a cake in so far as you are covering a soft background with a hard shell and this is why cracking occurs. Each plaster coat should always be softer than the previous. Also where a gypsum or sand:cement backing plaster has been patched in next to lime plaster, where the hard and soft edges of the two plasters meet is a likely place for cracking. It is worth mentioning that gypsum based plasters and lime based plasters are not realy compatable although (for what ever reason) it is common practice for gypsum skims to be applied over lime work (with the liberal use of PVA as a bonding agent)

 

[monkey writer]

what about skimming over gypsum with lime? Would that be okay? - my walls have bits of both all over them (and some cement, concrete(might be) and what looks like it might be plaster of paris?).

 

[BMcD]

With a suitable bonding agent such as DG27 I see no reason why you can't skim lime over various backgrounds including plasterboard though you may only be getting an aesthetic benefit and to realise the full benefit of lime then you would be best off reinstating lime based plasters throughout. My advice generally would be to keep gypsum with gypsum and lime with lime

 

[Carol]

Two years ago we removed a 1m high x 3m long strip of internal plaster from our back wall that was causing damp problems and lime-rendered/plastered. This has worked very well although it was a 'temporary' solution. Aesthetically it is not brilliant as you can see a 'join' between the lime and the gypsum, but its better than blown holes. Problem is caused by modern finish/paints on old wall outside and awaits huge financial outlay to remove all old render and re-lime. Builder was also concerned as to what we may find below old render, as house began life as a timber frame, but has had some, possibly all external timbers removed on this wall. :?