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.

Stephen wrote:
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.

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.

On 18/06/14 23:05, Capitol wrote:
Stephen wrote:
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.

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.

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...

On Wed, 18 Jun 2014 23:05:48 +0100, Capitol wrote:

Seems like the cement render is too soft. I'd hack it off and redo with
3 to 1 sand /cement.

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.

--
Cheers
Dave.

On 20/06/2014 08:52, Dave Liquorice wrote:
On Wed, 18 Jun 2014 23:05:48 +0100, Capitol wrote:

Seems like the cement render is too soft. I'd hack it off and redo with
3 to 1 sand /cement.

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.


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

"Stephen" wrote in message
...
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.

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.

In article ,
Stephen writes:
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?

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

Would PVA be the way forward?

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.

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.

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).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 19/06/14 09:50, Andrew Gabriel wrote:

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).

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.

On Wednesday, 18 June 2014 22:49:59 UTC+1, Stephen wrote:


I have an internal solid blockwork wall with sand and cement rendering
The plaster has "blown" off.

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

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 usually. 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 and let it pout in down the brick-work. I don't know how well the pva will be 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 learned to copy and paste without how to think for themselves. And it won't be 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 thinned PVA on the surface will lay the dust and permit you to reskim it with little 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 have the patience to learn. Good luck.