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Painting old walls & new plaster - help a newbie out!

  • 19-08-2024 4:56pm
    #1
    peckerhead
    Registered Users, Registered Users 2 Posts: 3,382 ✭✭✭


    Hi all,

    So I'm planning to repaint a sitting room that has a mix of fairly rough old walls and some newer walls that haven't been treated at all since they were plastered over a year ago. Pix attached for to give you a rough idea. The alcove sections you can see are going to be largely hidden behind TV/bookcases afterwards so I'm not aiming for perfection there.

    Two main questions -

    A. After I finish sanding and filling/repairing the old walls, should I seal the surface with a thin coat of diluted PVA, or just water down the first coat of undercoat? (I'm painting everything white for now.) And what about the unpainted plaster? I'm seeing conflicting advice on the internet...

    B. That 'trench' where the electric cables were brought through for the sockets - what's the handiest (safe) way to fill it. Caulking gun? Polyfilla?

    Thanks in advance for any tips! :)

    IMG_20240819_155217.jpg
    IMG_20240819_155236.jpg
    IMG_20240819_163422.jpg

    Tagged:


Comments

  • #2
    Rows Grower
    Registered Users, Registered Users 2 Posts: 8,234 ✭✭✭Rows Grower


    Water down the first coat of paint for the walls and use either sand and cement, bonding or skim coat to fill the trench.

    "Very soon we are going to Mars. You wouldn't have been going to Mars if my opponent won, that I can tell you. You wouldn't even be thinking about it."

    Donald Trump, March 13th 2018.



  • #3
    mickdw
    Registered Users, Registered Users 2 Posts: 23,406 ✭✭✭✭mickdw


    I stick a bit of bonding in the big trench. It dries in minutes and will easily fill the larger gap. Finish off with filler. I tend to use bags of powder joint filler for any filling work. Goes off quick, and you can work it to a finish as it goes off.

    The powder filler and a wide paint scrapper is also useful for filling slightly rough wall as you can make it into almost a slurry and run it over areas that need minor filling and scrap surface clean immediately.



  • #4
    10-10-20
    Registered Users, Registered Users 2 Posts: 7,238 ✭✭✭10-10-20


    Hi! The best filler for much of that is stuff called bonding coat (and not "bonding") if you can get it in 12kg bags. It's light-weight and easy to work with. You can just dampen the void and then backfill it with bonding coat, but then leave around 3mm to finish it off with a skim or finishing filler.

    https://www.goodwins.ie/products/gyproc-thistle-bonding-coat-plaster-25kg.html?filter_set%5B%5D=1413,5908,5914

    Another option is going for a bulk filler such as this one

    https://www.goodwins.ie/products/proform-all-purpose-filler-5-ltr-0023332.html?filter_set%5B%5D=1413,5908,5913

    As you're intending painting the wall and you have a mixed substrate (differing plasters and finishes) then I'd use a surface prep such as ESP, except it's pricey for a job which isn't going to be seen:

    https://www.goodwins.ie/products/owatrol-e-b-1ltr-emulsa-bond.html?filter_set%5B%5D=8,2132

    I'm not a fan as PVA as a surface prep, it adds some problems with paint adherence as well as often being the wrong tool for the job. Watering down the initial coat (by around 20%) would be my preference if you wanted to keep the cost low.



  • #5
    standardg60
    Registered Users, Registered Users 2 Posts: 4,798 ✭✭✭standardg60


    I've never understood the thinking behind 'watering down' tbh other than a means to stretch the paint out for the first coat. It's just going to evaporate more so I don't see how it helps an already diluted product adhere more, less would be my thinking.

    Sealants form the best connection between the wall and the paint, that's why it's on the shelves. Anything else is false economy imo.



  • #6
    whizbang
    Registered Users, Registered Users 2 Posts: 1,943 ✭✭✭whizbang


    Is that a bit of mould there..?

    Also if there's old wallpaper paste, it can destroy many paint finishes. Not really sure how to cure that, but someone elso should help here.



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  • #7
    Ceepo
    Registered Users, Registered Users 2 Posts: 2,688 ✭✭✭Ceepo


    https://www.boards.ie/discussion/comment/122520661#Comment_122520661

    The idea of watering down the 1st coat, it to allow the paint to absorb into the fresh plaster. Applying paint directly could result in the pain only sitting on the plaster, which in turn will cause problems down the line. It has nothing to do with "stretching out" the paint. And it doesn't evaporate, it soaks in, the give a better key for further coats. Sealant are to seal if there's is an issue like chalky surfaces etc. You're not try to seal anything when applying the 1st coat, unless there's an issue.



  • #8
    peckerhead
    Registered Users, Registered Users 2 Posts: 3,382 ✭✭✭peckerhead


    Thanks all, much appreciated. I understand what people are saying about the "mist" coat on new plaster.

    Cleaning the ceiling with a bit of sugar soap and doing the filling and sanding tomorrow, hopefully first coat the day after. Will post back with a photo when it's done! 👍



  • #9
    standardg60
    Registered Users, Registered Users 2 Posts: 4,798 ✭✭✭standardg60


    https://www.boards.ie/discussion/comment/122523506#Comment_122523506

    Nope still don't get it. It's water, of course it's going to evaporate as the paint dries. Put it this way, what is the difference between wetting the walls with the water beforehand rather than adding it to the paint? It's still going to sink in with the result that the paint itself doesn't have to resulting in more coverage, but the water itself is going to act as a barrier to the absorbance.

    The sealer absorbs into the wall on the one side and bonds to the paint on the other, how can water do this? Is there any science behind the use of watering down?



  • #10
    Ceepo
    Registered Users, Registered Users 2 Posts: 2,688 ✭✭✭Ceepo


    https://www.boards.ie/discussion/comment/122525183#Comment_122525183

    To be honest I don't see what the issue is. This is the process that almost all professional painters use on new skim plaster, except when a specific issue arises and a different approach is needed.

    Diluting the 1st coat of paint is more of a primer and will allow the paint to absorb onto the plaster, it also acts as a sealer for further coats. If you apply the paint full strength the plaster will soak the moisture out of the paint like a sponge, which may cause it to crack, or just sit on the wall and not bind properly. I wouldn't use the term evaporation, as it doesn't really turn into a gas and disappear, more is soaks into the wall. I suppose there is nothing stopping anyone using a specific sealer like Polybond/Durabond, stabilizing solution, some of the Zinzzer products or even dry wall primer that specific for taping and joining walls, but unless there is underlying issues like powdery or flakey surfaces there no need too. Plus you have the added costs of buying the products.

    If you wet the walls before hand, then you are applying paint to a wet wall that may have different amounts of moisture, as a result it will be much harder to ensure you get an evening finish as the roll will skip/skid on surface. This is before you consider the added mess of having water everywhere.



  • #11
    10-10-20
    Registered Users, Registered Users 2 Posts: 7,238 ✭✭✭10-10-20


    Ceepo's right, the extra water helps the paint draw into the fresh plaster and causes it to bind. That reduces the 'suck' (suction) on the subsequent coats and the paint then binds better to the wall.

    If you just use paint alone then it can just sit like a drape in a thick layer on top of the plaster as the water gets pulled away from the paint layer closest to the plaster and never binds in. As it's not bound to the wall its weak and deforms or cracks when knocked.

    t's the same reason why you have to also control the suck with plaster on some surfaces. New plaster over old won't bind as the plaster layer dries too quickly and then shrinks and pulls away and leaves a void. You'd hear a hollow sound if you knock it afterwards.

    A very watery mix of PVA (1 to 4 of water) would probably do the job if I'm honest, I'm just dead against it as it's often used in the wrong location and can cause issues.



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  • #12
    Ceepo
    Registered Users, Registered Users 2 Posts: 2,688 ✭✭✭Ceepo


    https://www.boards.ie/discussion/comment/122525925#Comment_122525925

    Tbh, id stay form PVA altogether as it doesn't allow the walls/plaster to breathe. If the walls porous there are better alternatives. Applying PVA before was a big thing 30 years ago but it quickly became apparent that it was causing issues, unfortunately the myth that it was a the greatest thing carries on .

    @standardg60 . Have a read of this article. There are 1000s more like them if you don't want to take my word.

    https://coatpaints.com/blogs/how-to-paint/how-to-paint-on-new-plaster?srsltid=AfmBOopAvi0ltyIhKN6umuaf958BIAMGBw6M5ynD9K1OrNOwJFX4DeQB



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