New plaster not sticking to wall (sand and cement)

[bp]

Hi,

I am attempting to apply a top coat of plaster onto a smallish area (not the whole wall) where the top coat has come away due to renovations work.

I am using Finishing plaster and following the instructions on the back.

The base coat already on he wall is a sand and cement based render. I apply the top coat(skim) approx 2-3mm thick. It goes on OK, but once dry, cracks appear int he new plaster. It aso does not bond well and just comes away.

I think the sand and cement coat is sucking all the water from the new plaster.

Whats the soloution. Removing the sand and cement coat is not an option at the moment.

Thanks

Bhupesh

[mark.hannah]

Brush on a coat of PVA to seal the area and stop it absorbing all the water from the finishing plaster?

Mark.

[mark.hannah]

Also perhaps score the sand/cement coat to provide a key for the top coat?

[bp]

OK,

Thanks for the replies.

2 more questions about PVA.

Could I use any PVA. I have some childrens PVA glue type stuff. Or should I get some specific for plastering - which I guess I could get from BQ.. I would say I only need a little bit though

How do I apply it. Thinned (with Water?) Or direct. How long do I wait after applying PVA to apply the plaster?

Many Thanks

Bhupesh

[FKruger]

You should paint on PVA just before plastering. The PVA should still be a bit sticky as you apply the finish. I would also paint a dilute coat of PVA on the day before to reduce the suction.

[The Natural Philosopher]

no. Soak the wall that you are plastering first.

And score it too.

[FKruger]

PVA is about £10 for 5 Litres at Wickes. Small quantites are going to be pretty cheap so may as well use the correct thing.

I just paint on the PVA neat, then I mix the plaster which takes a few minutes then I apply it over the PVA straight away.

[John Rumm]

Yup, played that game!

Dilute PVA (about 1 part glue to 4 water ought to be enough), apply a generous coat will a roller, and plaster straight onto it. That ought to stop the wall sucking out.

[bp]

OK,

I have followed the advice of the postings here. This is what I did:

PVA diluted and applies 24 hours beforehand. Then when ready to plaster I apply PVS neat to the wall, waited to get sticky then plastered on top.

I still see the plaster cracking. About 30 mins after, I see the plaster cracking.

What am I doing wrong. Could it be that the area I am plastering is too small for the type of plaster that I am using. I am using Finishing plaster. The areas I am plaster are mostly small. The largest is about the size of a tea saucer. There are about 3/4 of those and the rest is at ground level where the top coat has come away when removing the skirting. There are also lots of small holes (a few inches square), where the dado rail was glued onto the wall.

There is cracking in all of the holes

I have never done plastering before, this is my first attempt. Should I be using filler instead of plaster?

Many Thanks

BHupesh

[Stuart Noble]

Diluted by how much? 1 pva: 4 water is about right.

Then when ready to

PVS? If you mean pva, don't use it neat. You shouldn't need this step at all

, waited to get sticky then

If it's cracking after 30 mins, I'd say you haven't used enough water in the mix.

I would leave what you've done to dry. It's nearly always a 3 day process

Day1: Pva as above. Fill the hole with whatever plaster you have lying around and get it level with the wall. Run something straight across the hole to make sure it's not proud of the surface, but don't fuss over it.

Day2: Run the back of a hand saw or similar over the hole to knock off any bits and pieces of plaster. Use a plasterers trowel to skim the surface with filler. Don't worry about getting it all round the hole as long as it's flat and smooth.

Day 3: Sand it smooth. If you've done stage 2 well, you may not even need this.

[Maris]

I agree with the above. Forget the PVA. Obviously it should not be running with water!

[John Rumm]

You could give the wall a good spray with water first if it is really absorbant. Then 1:4 PVA, then plaster. Use MultiFinish plaster (not finish or board finish) as it is better on high suction backgrounds.

Mix it to a wet consistency - like softly whipped cream, so that it just about avoids sliding off your trowel when you turn it upright.

Don't over work it - apply smooth it a bit and then wait until it starts to go off. polish a little bit and then leave alone again. Wait until it dries more. For a final polish splash with a little water first.

You may be better off with polyfiller for that size.

[Andrew Gabriel]

They are a bit small. Plaster is difficult to use in small areas.

Finish coat shrinks as it sets. You should have something like

1 - 2mm thickness, in which case it shrinks by getting thinner. Any more than that and it may also shrink by cracking. If you need to apply more than this, do one coat which brings you to 1mm short of the final surface (doesn't matter if that cracks -- it provides more key for the finish coat), and when this has nearly set but is still wet, apply the finish coat.

When you mix it up, it should be very sloppy. A dollop on a trowel should run off if you hold the trowel face vertical. If you are using it as a filler (i.e. for deeper holes), you'll need to mix that up thicker, as it will otherwise fall out. Finish coat doesn't make a good filler, but it can be handy to avoid getting a bag of something else as well. However, it must be sloppy/running for the finial finish coat.

[Andrew Gabriel]

One other thought -- you might have a sand and cement scratch (under) coat to provide a moisture barrier. In this case you mustn't fill holes in it with gypsum plaster, as that will wick water straight through from the back.