skim over plasterboard and one coat?

[ob0t]

Have boarded over a void in a brick wall, and now I'm trying to finish the surfaces off. The brick wall itself is plastered to about 25mm depth, although there's a big chunk been hacked off that's exposed the brickwork underneath. The back of the board is pretty much level with the brick face, so I need to bring that out about another 10-15mm, and restore the exposed bricks to the same level.

Untitled

• ob0t
• 1 Oct 2009
• 1

Last weekend I did a layer of one coat on the bare bricks to bring that up to the same level as the board. My plan was to do a couple of layers of skim over that and the boards to level things off.. Is this likely to work, or do I need to pack the one coat and board out a bit more first?

Or maybe I'm barking up the wrong tree altogether...?

 

[Richard C]

The problem with covering such an opening with PB is the very high risk of cracking in the join between the original wall & the new plasterboard but there are methods that will minimize the risk considerably. Can I first ask how you’ve supported the PB covering over the void? From what you have said so far, you may not have made things that easy in what youve done!

 

[ob0t]

the boards are secured to a timber frame built into the void, which is fixed to the existing brickwork on all sides.

thanks for any advice!

 

[Richard C]

What I normally do is remove around 300mm of the plaster around the opening & fix the PB both to the studwork & the brick face. Make sure the plaster has a raged edge as this provides a better key & gives a better chance of avoiding cracks in the future. You’ve left yourself a large difference in plaster levels to make up & hanging 12mm of plaster onto the face of plasterboard is not ideally, normally it would only have around a 3mm skim coat. You need to get the front face of the PB out flush with the surrounding plaster, fill the gaps between the 2, reinforce the join between the PB & the original plaster & re-skim the entire wall; done properly, this would make it invisible.

If you feel that’s to much for you & just want to blend in, position the PB around 3mm below the surrounding level but finishing plaster is not that easy to blend in & you will inevitably see the repair & without any reinforcing tape under there, it will very likely crack open again where the 2 meet.

The best way of proceeding would still be to remove the plaster around the opening & I would normally space the stud frame accordingly but, in your case, replace the bit you’ve already put up with a larger sheet & double up over the top using either 12.5mm or 9.5mm PB accordingly; use PB adhesive to bond the PB to the wall.

Personally I would never use one coat plaster, it has some uses but I hate the stuff. Bonding plaster would probably be the easiest for you to use as a base coat for any filling out as it’s slower to go off but if you have no experience, getting a decent finish on the skim coat will probably be beyond you unless you practice on a couple of boards first. Skimming the entire wall is the best solution but if you’re not confident you could do it, it may be best to get a plasterer in to do both the prep & skim as it won’t really take him much longer & the cost difference won’t be much.

 

[ob0t]

Good stuff.. No problems bringing the front of the frame flush with the existing plaster. Happy to give the plaster jobs a go and get a pro in if/when I screw the whole thing up...

Just to recap, then - strip plaster back a foot, fix PB with the front flush against what's left, fill the gap, tape across the join, skim the lot (with multi-finish?), then second skim a couple of hours later.

A few questions on that lot and (hopefully!) I'm ready to go:
- Any tips for removing the mortar/plaster section without damaging the brickwork and/or the whole lot coming off? Just chip away at it with a bolster chisel?
- What to fill the gap with? If the gap is quite small, then would something like expanding foam do the job? Presumably if it's large then I'm looking at filling with bonding/one coat anyway..
- Existing plaster wall has been painted - can this just be primed with PVA before skimming or does it have to be stripped first?

Thanks again for your help. Really can't believe how much helpful information there is here!

 

[Richard C]

Just to recap, then - strip plaster back a foot, fix PB with the front flush against what's left, fill the gap, tape across the join, skim the lot (with multi-finish?), then second skim a couple of hours later.

That’s basically it but you must bond the PB to the brickwork as well as th stud. Use 3 overlapping layers of reinforcing tape over the join. As you will be skimming different materials, give the whole wall (including the PB) a couple of sealer coats of PVA/water (1:5) & let it fully dry out for 24 hours. Give the wall anther PVA coat (1:3/4) when you’re ready to go & plaster when tacky. 2 hours is way too long for the 2nd skim coat & it must go on just as the first starts to firm up. The two coats are best applied from the same mix &, on an average size wall, it should be ready for the 2nd coat more or less as soon as you’ve got the 1st one up there & basically levelled. If your inexperienced, you will really struggle for time so probably 2 mixes will be better for you but timing is very important & you must be quick; if you let the 1st coat go far it’ll defeat the whole point of 2 coats & it'll probably end up looking like a ploughed field.

A few questions on that lot and (hopefully!) I'm ready to go:
- Any tips for removing the plaster section without the whole lot coming off? Just chip away at it with a bolster chisel?

I just use a bolster chisel to mark a line around the opening where I want to remove the plaster; a solid whack will crack the plaster & render underneath, then just remove the plaster from the edge, again using the chisel, & it should crack off at the line but if't loose or sounds hollow you must take it back further until it's solid. Go around the edge of the plaster with a hammer to break it away so it's not a distinct line; the ragged edges provide a good key.

- What to fill the gap with? If the gap is quite small, then would something like expanding foam do the job? Presumably if it's large then I'm looking at filling with bonding/one coat anyway..

Don’t use expanding foam only Bonding plaster.

- Existing plaster wall has been painted - can this just be primed with PVA before skimming or does it have to be stripped first?

If it’s ordinary matt, it should be OK. If it’s vinyl & particularly sheen or gloss, 2 ways to prep;

1. closely criss-cross score the whole wall right through the paint layers with an old scraper or trowel & give it a good going over with a very stiff wire brush before the initial PVA.
or
2. mix up a slurry coat using PVA & some bonding plaster or some cement & brush this onto the wall, once dry it will leave you with a fairly rough surface to plaster over.

Be warned, plastering isn’t as easy as it may look & in most cases your 1st attempt will end on tears! I would advise you at least have a practice session on a sheet of PB before attempting the wall itself.

 

[ob0t]

Be warned, plastering isn’t as easy as it may look & in most cases your 1st attempt will end on tears! I would advise you at least have a practice session on a sheet of PB before attempting the wall itself.

Point taken. The bottom half of the wall will be covered with T&G paneling next month, so sounds like that'll be a good place to start - if I make a complete hash of that then it sounds like it's time to get the boys round to do the bit that actually matters.