Plastering.......

[Stephen]

Hi

I have an internal solid blockwork wall with sand and cement rendering on it. On top of this is finishing skim plaster.

The skim plaster has "blown" off. So I have chiseled all the skim plaster off. This has left behind the sand and cement render.

I am finding that the cement and sand render is "dusty" and that a groove can be made in the render when rubbing a single finger in it.

My question is whether the exisiting render is OK to support a new coat of skim plaster or have I got to hack off the render back to bare brick wall and replace with a two coat plaster system?

Would PVA be the way forward?

If it matters, the wall will be tiled after it has been reskimmed or replastered so I need to support the weight of the tiles.

[Capitol]

Seems like the cement render is too soft. I'd hack it off and redo with

3 to 1 sand /cement. Followed by a topcoat of plaster. That way I know it's going to take the tiles. You could get away with out the finishing plaster if you get the render smooth enough.

[harryagain]

You can PVA it but you will need lots as long as you existing is well stuck on (does not sound hollow when tapped) Several coats, the first couple thinned with water. Keep at it until you have a hard surface. Try a small test area first. Mix some PVA with your skim coat too (ie mix it with the water you use to make up the plaster.) There is special cheaper PVA for this purpose. Often has instructions on useage.

[Tim Watts]

Or do what I did to stabilise some floor screed that was also soft, which was to soak it in diluted SBR.

However, as you won't get such good penetration on the wall (because gravity is against you and you cannot pour it on by the bucketfull like I did), it still leaves the question that even if you make the surface

1/4" hard as iron (which SBR will do much better than PVA), will the rest still cope with the extra shear force of tiles.

You might be OK. But for a guaranteed result, go with Capitol's reply.

It's a risk vs buggeration call...

[Andrew Gabriel]

It needs to be bonded to the wall. Tap to make sure it's not hollow.

Yes. Start with it well watered down, so it soaks in as far as possible, no more than 1 part PVA to 5 parts water, but possibly even weaker. Let this soak in as far as possible, and then let it dry. This will bond the surface back as far as possible, and also reduce or kill the absorbancy. When you are ready to plaster it, mix up some 1:3 PVA and paint this on. If it doesn't soak in, leave it for half an hour to partially set, otherwise continue immediately with finish coat application. In eiter case, the finish coat is applied with the PVA not set, so it bonds to it.

You can tile directly onto render, but it depends on the flatness and stability of the surface. If the tiles will be wet (e.g. a shower), the render is not harmed by any water, whereas gypsum plaster will slowly dissolve in water. If you decide to tile directly onto the render, treat it as above but instead of PVA, consider EVA (usually marketed as Exterior PVA) or SBR (these will better survive any moisture which penetrates the grout), and you will probably need to wait until it's completely dry before tiling (check what the tile adhesive says).

[Tim Watts]

Also most tile adhesives (cementious and ready mix tubs) love SBR as it leaves a slightly tacky surface.

The only impact is if using non absorbent tiles (glass, certain stone) is that the "grab" of tub adhesive may be buggered up as there's nowhere for the water to go out from the adhesive as SBR seals the surface very well as is itself pretty moisture resistant.

Not a problem IME with "normal" ceramic tiles - they have enough dry biscuit to work well.

[Dave Liquorice]

Flippin heck that's a strong mix, is this a nuclear bunker? B-)

Provided the soft stuff hasn't blown as well, the couple of coats of PVA/SBR as already mentioned will suffice to stabilise the surface before skimming.

[stuart noble]

Ready mix tile adhesive makes a good skim. Lightweight and sticks well to pretty much anything. Whatever you've got lying around really

[Weatherlawyer]

What caused this to fail?

Water ingress? Was the original wall built during a builder's strike or some outrage where cement was in short supply?

When portland cement rendering took over from lime plaster, they used an 8 to 1 ratio but these days it is a lot stronger. OTOH they use brick sand us ually. A bit swings and roundabouts. The mix will affect how it weathers as will the stuff used as a plasticiser or hardener.

So first thing is how does wet weather affect it?

You might get away with adding dust sealant to glue it together or silicon resins for a more expensive option. scratch out a groove high on the wall a nd let it pout in down the brick-work. I don't know how well the pva will b e filtered out of the solution you use but sand is an effective filter.

Some of the suggestions on here might well be offered by clowns who have le arned to copy and paste without how to think for themselves. And it won't b e costing them a penny.

I wonder how well a drip feed would work. Lots of drips on here. And a lot of experts too. Yer pays yer munee and yer takes yer risks. Spraying thinne d PVA on the surface will lay the dust and permit you to reskim it with lit tle problems if the cement is still attached to the wall.

That's about all I'd do with it. Plastering can be quite relaxing if you ha ve the patience to learn. Good luck.