suction control

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church

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Suction control is one of the most important things a plasterer will learn. 99% of all background substrates will have varying degrees of suction. Most amateur/DIY plasterers will be caught out by background suction, most will put it down to not laying the plaster on the wall quick enough, but most of the time its down to lack of knowledge and poor preparation.

What is suction? At its basic level the substrate will pull the moisture out of the newly applied plaster, leaving it dry, brittle and unworkable in minutes. Most professional plasterers with some years under their belts will be able to determine the levels of suction from any substrate simply by looking at it. Of course we all get caught out from time to time.

How to control suction from walls and ceilings. Once you have determined that you need to control background suction its all about knowing what product will work the best, most plasterers will use PVA (Polyvinyl Acetate). There are loads of different brands on the market varying in price as well as quality. All manufacturers recommend diluting the PVA with water and using this as a priming coat. After application it will have to be left for a minimum of 12 hours to do its job properly, even though it could feel touch dry in 30 minutes. Some people recommend multiple coats, but in my opinion I do not recommend it. After the priming coat is thouroughly dry, most plasterers will use another coat of PVA called the Bonding Coat, this again is watered down but not to the degree of the priming coat, this again is applied and can not be plastered onto until the PVA feels "touch tacky".

Controlling suction with primers and stabilisers. Again there is various brands on the market, British Gypsum make a product called Gypprime, yellow in colour and costs around £70 for 11 ltrs, this product can be diluted and British Gypsum state that it will do around 600 sqm per tub. It can be applied either with a brush or roller and even sprayed, but this is a professional product financially out of reach of most keen DIYers. Artex stabilex is another water based primer/sealer, which is white in colour and sells for around £12 for 5 ltrs, this product is one of my personal favourites. It also can be applied by a brush or roller and will work faster at sealing a substrate than PVA. There are also oil based stabilisers on the market but again they will need at least 12 hours to dry and these carry a powerful odour.Remember these products are only primers/sealers not a bonding agent thats a whole new topic in itself.

Last of all there is good old H2O yes water, although this will not seal a substrate and is not a bonding agent, applied in the right amount it will slow it down long enough for the plaster to be applied and is free! (well sort of).

The above information only touches on the beginning of what is to be learned about suction control, it takes a whole career in plastering to fully understand it, but I hope what I have touched on helps in some way.
 
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bigsegs

Guest
great post from a man with many many years of experience.. nice one church :)
 
B

bigsegs

Guest
Suction is your freind. Sometimes he can be the type of friend that 'just will NOT leave you alone', other times he just sits there in the background occasionaly adding his bit just to keep you going..
What am I on about?
Suction is what bonds your plaster to the wall, most building substrates are porous. Brick, block, finish plaster, as long as theyre unfinished (painted etc) then if you throw a cupfull of water at them it doesnt just run down the wall onto the floor, it SOAKS IN.
Then, in time, depending on how much water has gone in, it dries out again... This is whats known as 'breathing', so when people say 'the wall needs to breathe', it needs to be able to get rid of its moisture into the atmosphere.
WITHOUT suction you've got a problem if you want to bond ordinary finish plaster to the surface, youre going to need a key (see creating a key)
Now, if you want your new best mate to calm down a bit, i.e. slow down the rate/level of suction you're going to have to do something about it. This is whats known as 'suction control'
Too much suction will suck the water out of whatever it is your trying to cover the wall with (render, bonding, finish etc) and you'll end up something that looks like a crazy paved patio on your wall, brittle and unbonded.
Too little suction (without a mechanical key) and your material will set and cure on top of the surface, then fall off again..
my tuppenceworth anyway.. :)
 

phippsy333

Member
sometimes when u have no suction ie tiles or a gloss paint for instance you need to paint Gypsum bond-it / wicks pink stuff/ benkonatact to provide a key...but this shouldnt be used for killing suction as thats not what its designed for. (see creating a key)
Gypsum say u can use their Bond-it to skim on tiles..( as stated earlier)..me personnaly i have never done this as i feel your plaster is only as strong as its background substraight, and who knows whether the tiler has used the right adhiseive for the job and thus how strong the tiles ( being your background are)
 

kirk johnstone

Private Member
this might not be text book but it works well and its abit cheaper.
2 parts pva
2 parts water
1 part sand

one good thick coat, let it dry and away you go, like i say might not be text book but av never had a problem just dont forget to keep stiring it up as the sand will drop to the bottom :-*
 

Chris W

Well-Known Member
gonna have to argue slightly with the previous post and split some hairs...
pva is used for 'suction control', the reason you leave it to go tacky is if you lay on whilst its still wet youll just slide youre spread around the wall instead of the wall taking it off your trowel..
if you do have some suction, enough to use pva, letting it go tacky will also adhere the plaster to the pva and the pva will adhere to the substrate...

if your on a silk or gloss painted wall try this -
paint some pva onto a sheet of glass and let it dry for as long as you like hours, days, weeks, months whatever...
then get your trowel and try to scrape it off...
pva is an adhesive used where the substrate is porous, its fantastic stuff, that good in fact that kids use it to paste pictures into paper scrapbooks...

bonding agent is designed differently.. i.e. to bond to a substrate that HAS no suction...
do the glass test with some wba or thistle bond-it...

or when you use it, get it all over the place, windows, doors, ornaments, womans wardrobe etc and see how easy it is to get off again...
 
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