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PostPosted: Sat Aug 15, 2009 7:36 pm 
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chrisw w wrote:
greengrass wrote:
No need to devil a wall if using PVA. PVA glues the plaster to the wall IE; it's the key ;) Probably the wrong mix and applied wrong. I have proved the point before and plastered a sheet of glass after applying PVA.

try painting some neat pva to a sheet of glass and let it dry...
then try and get it off...
how easy was it to get off?
now try pva'ing some standard plasterboard and let it dry...
how easy was it to get off this time?
http://www.british-gypsum.com/pdf/DS-01 ... istant.pdf
theres a reason british gypsum specify thistle bond it to moisture resistant boards with no suction...
you wouldnt try and glue two bits of plastic together with pva.. youd use a polymer based adhesive such as bostik...
however, two bits of porous wood will go together lovely with pva and a bit of pressure..
worth thinking about...


The idea of PVA is to seal and bond when you have no key surface. Wasn't aware we were talking about damp surfaces..? To plaster a painted surface PVA will sufice and last for upto 50 yrs if done correctly.

Looking at the pictures I don't see flood damage and it's a stud wall so no rising damp, wondering where you got the damp idea from?

Am I missing something?


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PostPosted: Sat Aug 15, 2009 8:52 pm 
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i never mentioned damp at all, what im saying is that a moisture resistant board has little or no suction. british gypsum state thistle bond it be used first before plastering moisture resistant boards. The reason for this is there is little or no suction.
what bondit does is adhere to the surface in a way that pva cant, it sticks better. try painting 1/2 a sheet of glass with thistle bondit and half the sheet with pva. apply plaster to a sheet of glass and it wont stick. it'll dry and stay there but give it a whack and it'll fall off again. pva the glass and plaster it while tacky and the plaster will bond to the pva. only problem is the bond between the pva and the glass is very weak.
apply thistle bondit to a sheet of glass and let it dry. Then apply plaster to thistle bondit and it sets around the aggregate which is bonded to the polymer based adhesive which is bonded to the glass a bit like superglue.
some people swear by throwing a handful of sand into some pva and using that on areas of low suction. waste of time in my opinion, its all about the adhesive on a low suction background. Suction is what gives you a key under normal circumstances, the same way render sticks to bricks. you'd never get render to stick to glass, pva or not, too heavy.
just my opinion but ive looked into this extensively because ive had the old 'plaster falling off the wall' trick in the past and i found myself seeking the reason why, so i didnt make the same mistake twice.
ive also managed to get plaster to stick to silk paint with pva but i also notice that not all silk paint is completely waterproof...
i found this out to my cost the other week when i attempted to overpaint existing silk paint that had been on years only to find it blistered. It wouldnt have done if the silk was watertight... definately wouldnt have done with gloss paint...
ive also plastered a silk painted ceiling before today without any prep whatsoever, suction was near as dammit perfect, just like skimming board..
all depends on the paint, substrate etc...


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 Post subject: Another problem...
PostPosted: Sun Aug 16, 2009 2:26 pm 
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I've just found another problem in one of the alcoves. As you'll see from the photograph, the plaster has pulled right off the bricks, which appear to be painted. Any ideas on how this is best dealt with? The backing is an inch thick in places.


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PostPosted: Sun Aug 16, 2009 6:19 pm 
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mmmmmmm...
looks like a brick up job gone wrong that does...
maybe an old inset alcove within the alcove? just guessing by the paint and the row of headers along the top or even an old doorway? not forced to be the case though..


either way... the problem could be down to a few things...
be here all day explaining rendering but what i'd do is test the adhesion of the render some more i.e. if its loose/hollow have it off...
then youve got to wire brush the paint off, should come off easy looking at it..

difference between hardwall and render...

hardwall is prebagged, easier to use, more likely to stay put
BUT it will absorb moisture like a sponge...
if there is and has been no sign of damp and its not a solid nine inch wall (unlikely looking at it) i'd go for the wire brush and hardwall option but i'd leave it to the spread who does the skimming because it wants to be done same day...

if its possibly prone to damp i'd render it (sand and cement) with a waterproofing additive building it up in two coats scratching the first one really well and devil floating the second coat day before its due to be skimmed. first (scratch coat can go on a week earlier, in fact the bigger the time gap the better between coats of render..)


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