First Time Plasterer Seeking Advice

Discussion in 'Other Trades Talk' started by AndyF55, Feb 26, 2014.

  1. AndyF55

    AndyF55 New Member

    Second wall done. A bit of a rush job to be honest - started late and had to finish before pancakes ruined.

    I reckon I've not been making the plaster thin enough as it has been a right job to get the first coat on. I

    Also, working onto the dry PVA surface rather then when tacky did cause a few more difficulties. There were a few instances of the plaster pulling away on the smoother surfaced areas beneath.

    The issue of the adjacent untreated walls sucking moisture from the newly applied plaster certainly was notable - it make a huge difference.

    Two walls and a ceiling to go.
  2. BuilderMCR

    BuilderMCR Active Member

    The dry vs tacky thing shouldn't have made any difference, when the moisture from the plaster hits the pva it'll 're activate' the pva anyway (make it tacky)

    The smoother surface areas underneath probably had no suction.

    I always use betokontakt on re skims with no suction.

    You could try wickes bonding agent. It comes in smaller tubs than betokontakt so it's cheaper. A tub should do an 'average' size room
  3. Your are learning the right way, Andy - through trial!

    As MCR says, applying on to fully-dry PVA shouldn't be a problem as the surface will 'reactivate' with the moisture in the plaster. Just don't try spreading it out too thoroughly to start with - just get it on, trowel it out quickly and roughly as you would normally, and then give it a good 30 seconds or so before levelling it any further - ie: move on and come back to it. It should be 'stuck' by then!

    It should clear that PVA does have 2 purposes whether they are both actually required or not; one is controlling suction (when needed) and the other to provide adhesion on unsuitable surfaces. By its very nature, if you apply PVA just for the 'suction' reason, then it will also be acting as an adhesive since that's all the plaster now has to grip hold off - the surface underneath the PVA is effectively lost to the new skim in this respect.

    I guess if 'adhesion' is the main requirement, there are better - purposely designed - products out there.
    Last edited by a moderator: Mar 5, 2014
  4. Ah! That's what you plastering finish reminds me of, Andy - pancakes :p

    (Only kidding... :))

    If you have a wall to do that has a Southish-facing window on an adjacent wall, get ready to be alarmed. I did the new stairwell in our current house which has a Velux in the roof at the top. It all looks ok until the sun shines down almost flush with the wall. Jeepers... :eek:
    AndyF55 likes this.
  5. FatHands

    FatHands Well-Known Member

    The spread i worked with always told me to PVA the night before, then start plastering in the morning. That's what i have always done using quite a thick mix: 60 water, 40 PVA
  6. BuilderMCR

    BuilderMCR Active Member

    The suction is what causes the 'adhesion' lol

    The background isn't lost to the pva, which is why you dilute the pva.

    Walls with no suction that have been glossed you've got no chance of skimming it using pva. Which is why plaster bonding agents are used, to provide a key.
    Whether you use pva or not if you have no suction your skim won't last long.

    If pva is used for adhesion why do British gypsum not recommend pva?
  7. Hmmm, I think you are jumping to conclusions, MCR.

    'Suction' is what causes adhesion? No it isn't. Adhesion is a molecular process than bonds the 2 materials and works regardless or whether the surface is 'sucking' or not. I think you may be getting 'suction' and surface 'key' mixed up.

    'Suction' in terms of a surface to be skimmed means how much it removes moisture from the plaster - and you want that to be as little as possible - even none is ok. Plaster sets by a chemical process like cement, and adheres like cement too - it doesn't matter if you 'suck' it or not - it'll still set and stick to most surfaces (gloss paint being one of the exceptions - because it's smooth, glossy and designed to repel things sticking to it - not because it doesn't 'suck').

    And you are right - PVA is no good at priming a gloss-painted surface because PVA is a plastic and doesn't adhere to other plastics. Not because gloss paint doesn't 'suck'.
  8. BuilderMCR

    BuilderMCR Active Member

    Next time you skim a wall do as many coats as you can out of full tub of pva neat. See how well your skim holds with no suction.
    Remember to start a thread called 'sorry mr mcr, I was wrong' lol
  9. Tempted tho' I am to knock up a batch just to prove you wrong :)p), I'll have to resist.

    So, you are claiming what? That a single coat of PVA allows water to be drawn from the fresh plaster right through the PVA and into the wall behind? So that the wall does the 'sucking' of the plaster to it and that's how the plaster sticks? :eek:

    But a few coats of PVA won't allow water to penetrate right through so the skim won't stick to it...?

    MCR, I think we're going to have our first tiff over this :rolleyes:
    BuilderMCR likes this.
  10. BuilderMCR

    BuilderMCR Active Member

    Lol, no I'm not saying just a single coat, on more dry and porous surfaces then additional coats may be required. But too much pva leaves no suction at all.

    Diluted pva soaks into the existing plaster

    imagine a sponge with all the 'holes'
    If you sealed some of the 'holes' the sponge would hold less water, if you sealed more of the 'holes' it would hold even less water. If you sealed all of the holes the sponge would become waterproof and hold no water.

    Same with plaster, if you seal it too much to a point where there is no suction the plaster won't take to the surface.

    The point is, you need controlled suction which is what the pva helps with.
  11. BuilderMCR

    BuilderMCR Active Member

    This, is why British gypsum recommend using bond-it on moisture resistant boards before skimming. Because moisture resistant boards are moisture resistant (self explanatory lol) so there is no suction for the plaster to adhere to the boards.

    Same as when you use too much pva, it becomes moisture resistant. No suction means no adhesion.

    Which is where bond-it comes in on walls with certain types of paint that don't allow water to penetrate. Same again no suction no adhesion. So you have to use bond-it to create a key

    Again, different forum but this might explain further.
  12. BuilderMCR

    BuilderMCR Active Member

    Have a read of the white book too on the BG website... If you have a bit of time to kill
  13. Feb-Bond Blue Grit
  14. BuilderMCR

    BuilderMCR Active Member

    Same as betokontakt, wickes bonding agent and bond it.
  15. Sorry, MCR - I really, really, really don't think that's right.

    And not because I'm a plasterer - 'cos I'm not. Just because it doesn't make physical sense.

    That cove on the forum you linked to is confusing two things and assuming too much.

    He's saying that the reason you cannot skim on to a gloss painted surface is lack of suction. No it ain't - it's lack of adhesion; two completely different things.

    And this is pretty obvious when you consider that you can skim on to some surface which have zero 'suction'. Which a primed gloss-painted surface would have. Still absolutely zero 'suction' there.

    And the cove in that forum also claims it's lack of suction wot prevents being able to skim on to water-resistant board. No it ain't - it's because water-resistant board is - by it's very name - resistant to water and repels water. Ergo, anything water-based it gonna struggle to adhere to it. It has b****r-all to do whether or not the surface sucks water out of the skim.

    That should be becoming pretty obvious by now...

    When you coat a surface with PVA, that surface behind that PVA coating has been pretty much effectively isolated from anything you now try and apply to it. You will, from that moment on, be applying 'stuff' to PVA, not to whatever is behind it.
  16. BuilderMCR

    BuilderMCR Active Member

    Lol, I think we're going to have to agree to disagree lol

    I'm right though.

    You've pretty much just said what I'm telling you without realising lol

    I've been plastering over 10 years, 4 years I did nothing but plastering so I know my fair share!

    I'm going to email BG and ask for facts because it's something which is highly debatable across the interweb.

    What ever they say must be gospel because it's their product.
    Phil the Paver likes this.
  17. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Firstly, floating and setting in the same day means water is already present and wet plaster sticks to wet plaster very very well. No need for adhesive or suction prevention.

    Second, plaster sticks to paper like s--t to a blanket.

    Thirdly, how would you treat a brick wall previously painted with silk or gloss paint, if asked to skim it with a coat of plaster?
    You would possibly sand it, PVA it, and then plaster it.
    You would pva it NOT for suction prevention, but to aid adhesion.

    Mr. HandyAndy - Really
  18. BuilderMCR

    BuilderMCR Active Member

  19. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    What ever they say must be gospel because it's their product.

    Oh so many times I've heard that, and it is so wrong to believe manufacturers recommendations of 'their own product'.

    Mr. HandyAndy - Really
  20. MCR, you are undoubtedly the pro when it comes to plastering. I'm beyond novice... :oops:

    And happy to agree to disagree - even tho' we are, as you say, very close to actually saying the same thing :)

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