Re-plastering recently plastered room


Postby wrenton » Sun Oct 03, 2010 9:13 am

Hi there,

I am going to re-plaster a room I plastered around 18 months ago, mainly for practice but I also have a few issues.

1. The plaster has not bonded to a section on one wall (external brick wall). I plastered directly on top of a painted wall with plaster under it. The wall had been papered previously and we did seal it with watered-down PVA. The plaster has blown in one area about 300 x 50 mm. What's the likely reason for it not adhering? How do I now ensure a good finish this time? The wall has emulsion paint currently.

2. The other walls have no issues with plastering stick but there are quite a lot of hairline cracks along the corners of the newly plasterboarded walls/ceilings. How do I make sure the cracks don't appear this time?

3. A small section of ceiling got damp during building works and there's a small brown stain - will this re-appear through new plaster?

Any help (especially point 1) is appreciated.


James
wrenton
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Postby 30yearsinthegame » Sun Oct 03, 2010 1:08 pm

Hi

Your post is a little sketchy so I will try and give solutions to plastering instead of telling you what to do. You need to be more presise when talking about plastering ie skim or render.

If you don't take wallpaper off then don't use pva. Use thistle bond it, which is a green , sand and glue mix, make sure you leave for 24 hours before skimming. If you use pva then your simply lifting the paper from the wall which then won't be bonded to the wall.

Its best practice to remove any wallpaper. Sbr won't work very well, like pva.

As for the cracks it could be a number of things . 1. Poorly mixed pink 2: poorly applied 3: your surface is to porus 4: external heat can make plaster dry to quick.

Sollution: make sure plaster is mixed well and not over mixed known in the trade as cooked.
Make sure you apply evenly with a max of 4 mm. Two coat work is best. Make sure your serface is not too porus. Use scrim over exsisting cracks. You can use figreglass to reduce cracking.

Hope this helps and good luck
30yearsinthegame
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Postby wrenton » Sun Oct 03, 2010 1:18 pm

Would it be worth removing all plaster from the wall where plaster wouldn't stick? Is it best to apply a thin bonding coat to the brickwork?
wrenton
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Joined: Tue May 04, 2010 2:52 pm


Postby 30yearsinthegame » Sun Oct 03, 2010 8:43 pm

Hi

This is where I'm a little confused. Any blown render should be removed, if its an old wall use muck, if not render with plaster.

Hope this helps
30yearsinthegame
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Joined: Wed Sep 22, 2010 11:47 pm


Postby wrenton » Tue Oct 05, 2010 12:17 pm

Thanks for the help, & pologies for my low-tec description!

The wall where plaster hasn't stuck had wallpaper applied over skim when I got the house (built 1916). I removed the paper then applied a water and PVA mix to the skimmed walls then skimmed with two coats. There is an area where the plaster has blown and I can flake off the coat I applied.

My plan is now to remove the skim coat I applied then remove whatever is covering the brick work (assume skim and muck). Should I then apply bonding plaster and skim on top? That's the only system I've used other than skimming directly to plaster board.

Where corners have cracks, the skim was applied on a very hot day, in a rush and without much previous experience - so a combination of bad ideas. If I now apply a further two coats of skim, are those cracks likely to re-appear? Can I prevent it if so?
wrenton
Posts: 5
Joined: Tue May 04, 2010 2:52 pm


Postby 30yearsinthegame » Tue Oct 05, 2010 6:51 pm

Hi

Itd ok with your desciptions , you see plaster can mean skim or bonding.

Please use sand and cemtnt 4-1 mix no lime if down stairs if its up stairs use bonding , hardwall or my fav tough coat.

I don't think your skim stuck because your pva mix was too weak. 5-1 is about right but if your not sure what pva mix to use then go for a 3-1 to be on the safe side. Its hard to tell unless I'm there. There is a grip tip: I often do it and get great results mix your pva and water 60 % pva 40 % water making an almost neat mix. Cover the wall in the evening by doing 1 coat leaving it to dry for half an hour then another. Try and use a brush rather than a roller. Then when its all dry in the morning skim the wall. When liquid hits the pva wall the wall will be tacky leaving a great finish. But be quick this method will spead up the process of drying.

As for cracks they happen to the best of us. Tip: if a plastered wall is not painted withing 6 weeks it is prone to crack due to external temps. This is something people don't know. I wrote a class room paper once where we tested this and was sure right. I still have some class notes which I'm happy to email you, just let me know.

You can use firbre glass hair for re-skims and I get mine from rideons and this prevents cracks. Don't forget to use scrim on exsisting cracks. When you lay on your first coast put a little pressure on the trowel,

Good luck and come back and let us know how it went.
30yearsinthegame
Posts: 31
Joined: Wed Sep 22, 2010 11:47 pm


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