Plaster on different surfaces

Discussion in 'Builders' Talk' started by zulu100, Oct 24, 2006.

  1. zulu100

    zulu100 New Member


    I am a bit of a plastering novice but have got a bit to do and need some advice.

    I have filled in a arch in the fireplace with MDF to make a smaller square opening (Mrs will add dead sticks later!!)
    I know need to plaster the chimney breast to a surface to hang wallpaper on.
    I am begining to think i shouldnt have used MDF as i am not sure how plaster will take to it.

    I will use a PVA solution first to seal the surfaces but will that be enough to be able to plaster it?
  2. hallbeck

    hallbeck New Member

    Bad news mate - you can't plaster MDF. The water will cause it to expand and as it expands it will throw the plaster off. If i were you i would replace the MDF with plasterboard. For other surfaces:- remove all old wallpaper etc and then PVA walls with 3 to 1 mix
  3. Willy Duwitt

    Willy Duwitt New Member

    Bad news mate - you can't plaster MDF. The water will
    cause it to expand and as it expands it will throw
    the plaster off. If i were you i would replace the
    MDF with plasterboard. For other surfaces:- remove
    all old wallpaper etc and then PVA walls with 3 to 1

    Rather than replace the MDF, could he not just fix the PB to it ( using gripfill / no nails ) and then skim over that?
  4. devil's advocate

    devil's advocate New Member

    Hi Zulu.

    I think you'll get away with it. Apply a neat coat of PVA to the MDF and allow to dry fully. Then add a second coat; again allow to dry fully. Add a thrid if you want! Just make sure all the edges are sealed. This should be ok for a skim - I doubt if the water content will go through two layers of PVA. Apply just one thin (<2mm) coat of skim.

    Is the MDF a continuation of the breast - ie: is it butted up to the original hole so it gives a continuous flush surface? If so, have you bridged the gap with tape? If not, you'll almost certainly get hairline cracks along the breast-MDF join later on. If you want to add tape now, it might be worth removing a 1" strip of plaster (just as deep as the existing top skim) from the breast for half the tape to sit into so that the tape won't sit at a higher level which could be a nuisance. Seal the exposed plaster with PVA - in fact you can bed the tape down in the PVA while you're at it! If the chimney breast has a porous or uneven (in terms of porosity) surface, it might be worth going over it all with a single coat of PVA too - that way the whole surface (breast and MDF) should react the same to the skim, and dry evenly.
  5. devil's advocate

    devil's advocate New Member

    Allow all the PVA to dry fully before skimming.
  6. zulu100

    zulu100 New Member

    Thanks for the help chaps...

    I will give the PVA a go otherwise I think the easiest option instead of plasterboard is to talk the wife around to the arch again!!! then again.....

    When done will post if worked.
  7. mungo.

    mungo. New Member

    it wouldn't matter how much pva you put on it,its a big no no in my opinion,i would either overboard the whole front face of the breast with plasterboard,screwing into the mdf and either dot and dabing or no nails the rest then razorbead both sides and skim the lot
    hope that helps
    good luck with it
  8. devil's advocate

    devil's advocate New Member

    Hi mungo.

    I know it isn't the 'done' thing - and if I saw a pro doing this on my house I'd, er, have some things to say :). Hats off to yourself and the others above - I know you are saying the right thing.

    If it's easy for zulu to remove the MDF and replace it with plasterboard, then that's the best thing to do - using tape to cover the join. However, he sounds a bit like me - a 'ok' DIYer who sometimes starts a job and then thinks 'oops, perhaps that's not the best way; can I get away with it?!'

    Based on countless years of 'bodging' (;)) I think, if he coats all the surfaces, and seals the MDF back and front, there is every chance that it'll work. Obviously a bit of a gamble - but what's the worst that'll happen?

    The choice is his... :)
  9. cheltonian

    cheltonian New Member

    What an idiotic suggestion. You should not plaster on MDF whatever you treat it with.

    If you want to go ahead and bodge it you will never increase your knowledge or skill

    Overboard or replace with plasterboard then skim, as the earlier poster suggested.
  10. Chris Russell

    Chris Russell New Member

    I personally would rather rip out the mdf, dot and dab p/board in the void and scrim then plaster. But if you don't mind it falling off after being wallpapered then go ahead!
  11. dj.

    dj. New Member

    da, you utter utter ****, i rally am surprised at you giving such bad advice. never ever plaster over wood just by treating with pva. pva isn't the fix to all the problems you may like to think it is. as previously said, board over or take it off. do not skim over mdf!!!
  12. dirtydeeds

    dirtydeeds New Member

    the idiot armchair diyer suggested plastering over MDF is talking ******** to put it VERY bluntly

    you cannot plaster over wood, MDF, chipboard or any wood based material, there is no grip. AND even if you did the plaster would fall off because wood and wood based products move at a different rate to plaster.

    why do you think plasterers fix expamet to timber before plastering or rendering
  13. devil's advocate

    devil's advocate New Member

    What a tetchy bunch :).

    I fully accept it is not the proper way to do the job but, by the same token, I think there is every chance it would actually work in this situation.

    Why? Because by sealing the small piece of MDF it'll actually be pretty stable - a small piece this size ain't going to move very far. Will the plaster adhere? Too bludy right it will. No, I don't think that PVA is the panacea that some suggest, but it is damn useful stuff in the right situation.

    I'm guessing from zulu's description, we are talking about , say, a 2-3" strip of MDF which is a continuation of the fire opening. Provided it is fixed securely in place, I DO think there is every chance it'll be ok. The worst that's likely to happen is a hairline crack.

    No, it's not the right way to do it, but I'm also not the type to suck in air through my teeth and declare IT JUST WON'T WORK simply because it 'ain't the way to do it'.
  14. layiton

    layiton New Member

    Probably agree with devil there! Its definately not common practice but if its fixed securely primed properly edges scrimmed properly then skimmed properly the chances are you'll get away with it. If you do go down this route just square it with the customer that its the cheap option and could possibly create hairline cracks. Also about plastering onto wood, its not strictly true that you cant plaster onto it. laths(wood) plasterboard(paper-which i think is made somehow from err! wood!
  15. devil's advocate

    devil's advocate New Member

    Hi layiton.

    Can I have your baby? :)

    I'm actually assuming, from zulu's post, that he's doing this job for himself, in his own house (reference to the 'mrs' - I guessed his?). If he had been a pro, doing this as a paid job for someone else, I'd have kept my mouth shut :(
  16. I just emailed NASA and they said it's fine, they give the Space Shuttle a good half a dozen coats of PVA before and after all the ceramic tiles are fitted, so plastering a bit of MDF in a fire place shouldn't be an issue. It also acts as a heat shield as well as stiking the tiles on, amazing stuff.

    In fact I'm amazed this wood glue isn't used extensively in the building industry as a cure all for all and every problem...oh hold on, it is.
  17. odesymonds

    odesymonds New Member

    I had 2 loose bricks in the top of my garden wall, I stuck them in with PVA about 5 years ago and they haven't moved since. (as recommended by a friend in the building trade)

    (Beats mixing up some cement for a simple girl like me

    Ode xx
  18. devil's advocate

    devil's advocate New Member

    Phew! That's very reassuring Mudster; I was beginning to doubt myself.

  19. Sarcastic in here too are we?

  20. devil's advocate

    devil's advocate New Member

    Moi? Non. Well, un petit bit...

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