small plaster repair

[arl]

Im trying to repair some small holes in my lime plaster wall. Following the instructions in my DIY bible ive removed a small square of plaster around the damaged area and am now down to the brick. im wondering what to do about the edges. the top coat of plaster is sound and holds the square shape ive cut in it, but the hairy underlayer is very crumbly. Its adhered to the wall well but the edges are lose and powdery. What can i use to seel this (i was thinking PVA?). Should i use lime plaster or can i get away with using gypsum on such a small area?

Thanks in advance

 

[MDE]

Brush away the dust and use lime plaster.

 

[arl]

Thanks for responding so quickly.

ive tried brushing the dust away but it just keeps forming. it is so powderey i am brushing the bottem layer of plaster out leaving the the top coat floating above the brick. My small hole is getting bigger and bigger!

 

[Nigel Watts]

The important thing is that the old plaster must be thoroughly dampened before you apply the new. The suction created by the water will reduce the dustiness and allow the new plaster to grip.

After applying the new plaster keep checking it for shrinkage. If cracks occur as it dries, press it in more.

 

[BMcD]

Use lime based plasters. Cut back the existing plaster to where it is adhered to the brickwork and to as solid and square an edge as you can acheive. Stabilise the friable edge with a PVA solution or an SBR solution. Prepare the brickwork, if it is porous with excessive suction then it should be thoroughly dampened, if it is dense and smooth with poor suction then you can rake out the mortar joints to improve key, this has a disadvantage of a greater thickness of plaster within the mortar beds increasing risk of shrinkage cracks. Another method is to apply a spatterdash coat which is a stong, gauged, coase plaster slurry thrown forcibly onto the dampened brickwork and allowed to dry (it should resemble Tyroleane). The disadvantage with this is extra drying time. Once your background is prepared you should now apply your 1st. coat of coarse stuff, to about 8-10mm thickness. This can be gauged with 1/3 casting plaster. When this has picked up a little, scratch the surface with a pointed lath (or stick) to provide a key. The scratches should be made diagonaly in both directions so's to resemble a diamond lattice pattern about 25mm from centre to centre and not too deep as to cut through the plaster but deep enough to provide an adequete key. A comb scratcher is not suitable as it dosen't allow enough width in the score for the 2nd. coat. After the 1st coat has dried the 2nd. coat of coarse stuff can be applied, this too can be gauged with casting plaster. It is done in the same way only after application it should be ruled 2-3mm below the existing to allow for expansion of the casting plaster and depth for the finish, then scraped back if required. There is no need to scour gauged coarse stuff as this will kill the plaster giving a weaker set but the surface should be left rough or lightly scratched with a comg scratcher. If only a small area is to be repaired and is no thicker than 20mm then this may be roughed out in one coat as per the method for 2nd. coat.
The setting coat of fine stuff also gauged can now be applied after the float coat has hardened and shrunk sufficiently. This is then ruled flush with the existing and when firm trowelled off and fiinshed wth a semi wet brush. The joins should be troweled flush and any excess brushed from the existing to leave a smooth join.
NB. All surfaces, prior to plastering, should be dampened with clean water. They should be damp but not saturated or have water sitting on the surface. In hot weather or for porous backgrounds damping as you go may be necessary. A pump up garden sprayer is excellent for this.
It may also be necessary to dampen work down to prevent it drying out too quickly

 

[biffvernon]

Gosh. I'd just chuck some water at it and then fill the hole with 3:1 sand, lime mix.

 

[arl]

Thanks for taking the time to respond so fully.

I should have said it is really a very small hole (20x20cm). Im not sure it will be possible to apply layers of plaster in such a small area, and i dont really want to make it any bigger.
Will lime give me a better result than gypsum? I appreciate the use of lime for large areas for breathability (i even have the video) but for such a small repair is it necessary? Im worried too much about authentic period work (its a standard terrace that has already taken a real 20th century battering ie UPVC windows and missing fireplaces) and im sure the next occupants will probablly rip out all the interior walls to make it "openplan" or something.

If lime is better, does anyone know where i can buy small amounts in the northwest?

 

[biffvernon]

Yes lime is better - and easier to use than gypsum because you don't have to hurry. You can stop for a cuppa and it won't have gone off when you get back. I don't think I've seen it in less than a 25kg bag for around £7 so if you only need a little you've either got to scrounge some from someone who is using it or keep the spare for improving your vegetable patch.

 

[biffvernon]

oops

 

[Craig]

Maybe its possible to buy Garden lime in smaller bags?

 

[arl]

ermm, sorry, i may be getting confused but dont i need matured lime putty rather than lime powder?

 

[Craig]

Isnt it all the same stuff - maybe just depends on whether you can be arsed to mix it up.

 

[biffvernon]

Dry powdered hydrated lime from a builders' merchant will do just fine for your repair job.

It's almost the same stuff as wet putty lime. Like Chateaux Neuf du Pape '84 is almost the same stuff as Bulgarian Bulls Blood '05.

 

[arl]

Thanks for the advice. I shall crack on with it just as soon as the weather cools down a bit.