Another plaster repair question

[randers]

Hi. First post (not of too many I hope!).
I looked for the answer to this in other threads before posting but couldn't find it so apols if it's been asked/answered before.

I'm about to redecorate a couple of rooms, and can see some small areas of plaster has come loose behind the wallpaper. I THINK from reading other posts that what In should do is remove all the loose plaster, excavate any cracks, then am I right in thinking that....
- where the hole is deep apply 5:1 waterVA mix, then bonding, then a plaster on top? If that's right what plater on top - multifinish? and can I use a sand/cement mix instead of the bonding? What sand/cement ratio?
- where the hole is shallow just a plaster to fill - is multifinish best?

Thanks for any advice everyone.

 

[PrenticeBoyofDerry]

I'm about to redecorate a couple of rooms, and can see some small areas of plaster has come loose behind the wallpaper. I THINK from reading other posts that what In should do is remove all the loose plaster, excavate any cracks, then am I right in thinking that....

yes

- where the hole is deep apply 5:1 waterVA mix, then bonding, then a plaster on top?

You would prime all areas that are to be repaired, base plaster such as bonding coat can be used finishing with a multi-finish, a one coat plaster could also be used to substitute both bonding and multi, depending on the scale of repair?

If that's right what plater on top - multifinish? and can I use a sand/cement mix instead of the bonding?

Yes-Yes

What sand/cement ratio?

http://www.lafarge.co.uk/CementProductBrochures/Rendering_Packed_Cements.pdf

- where the hole is shallow just a plaster to fill - is multifinish best?

The area will still require priming, best to use a two coat method, multi is not the best filler and anything greater than 4-5mm holes it will better using other products such a bonding coat and one coat.

 

[NickB_99]

PBOD has covered it - only suggestion from a DIY angle is that easifill is probably more straight forward for them to work with than multi-finish.

The quantities needed are probably small also, and easifill can be bought in 1kg sachets e.g.
http://www.diy.com/nav/build/buildi...-Filler-5-1Kg-Sachets-11877570?skuId=12388276
Might help out the OP, as a 25kg bag of multi may be too much and once opened has limited life.

 

[randers]

New Question I'm afraid - one of the cracks I excavated was deep, and getting all the loose stuff out has left me a hole up to 5cms/2" deep in one place, right down to the wall construction. I bought a one coat plaster (plus some Gyproc Easifiller for shallower cracks elsewhere). The one coat says max depth 11cms. Is the performance of the stuff good enough to use it on that depth of hole if I build it up in multiple layers? I originally went off the sand/cement mix because of the 7-day drying time mentioned in the Lafarge sheet you posted a link to. Thanks again!

Also, the Gyproc bag says not to use over air-drying products, so I assume it's not an option over sand/cement. Is it OK over a diluted PVA film?

 

[PrenticeBoyofDerry]

Building one coat up to the depth required is possible. I would be tempted to use a reinforced mesh repair to a crack of that size!

 

[roy c]

Don't forget that the depth of one coat will take about 7-8 days to dry out if not more!!!

 

[randers]

Thanks again PBoD. I see Wickes do a reinforcing mesh (glass fibre) I assume that's the boy for me.
Going to have to take a radiator off the wall to get at the damaged wall properly....no doubt it'll drop to freezing while the living room's got no heating!

 

[randers]

Don't forget that the depth of one coat will take about 7-8 days to dry out if not more!!!

Hi Roy. The bag that the one coat came in says 1.5 hours drying time - so you've got me worried now. I assume I can just believe what it says on the bag?

Plus the mesh …. if I build it up in layers is that best put in on the first layer, or better to put it in at the end, below the final layer?

 

[roughcaster]

Randers,,, the 1.5 hrs mentioned on the bag means "setting time",,,, from the time you mix up the plaster, until the time the plaster sets/goes off,, 1.5 hrs or so.
The 7 or 8 days Roy mentioned will be "drying out time".
Setting time: 1.5hr,
Drying out time in this case: 7 or 8 days.
Drying out times will always vary depending on thickness of the plaster, the substrate it's on, plus the temperature of the room you're in. Plaster/cement etc is always better drying out naturally.

 

[randers]

b*****r! Thanks for letting me know tho else I'd have put on 4 or 5 layers in a day.
I'll change my plans then back to using a sand/cement render which if I haven't misread the Lafarge info can be applied to the full 5cm depth in one go and will take 7 or so days to dry. Please tell me if I'm wrong!
(Or did I misunderstand that each layer would take 7-8 days to dry and it's only the full depth which would take 7-8 days?).

I need to wallpaper over the top of the repair so I assume that if I put one coat plaster on top that'd take another 7 days, so is Gyproc OK on top of sand/cement instead and would that dry through quicker?

If not, as the above seems to take a month to repair the deep and quite wide hole, what else can I use to repair the hole that'd be quicker? E.g. are 'deep hole fillers' from e.g. Wickes any good?

And finally... I'm really sorry for all the questions... do I put on a dilute PVA before + after any or all of these coats?

Thanks guys, I really need the help as you can tell

 

[NickB_99]

b*****r! Thanks for letting me know tho else I'd have put on 4 or 5 layers in a day.
I'll change my plans then back to using a sand/cement render which if I haven't misread the Lafarge info can be applied to the full 5cm depth in one go and will take 7 or so days to dry. Please tell me if I'm wrong!
(Or did I misunderstand that each layer would take 7-8 days to dry and it's only the full depth which would take 7-8 days?).

For 5 cm depth, you do need to build up in layers. If you try that depth in one go, the chances are either render or one-coat will slump. You may not need 4 or 5, but certainly 2 or 3 are advisable.
You don't need to let each dry out for 7 days before applying the next - it just needs to be a stable base before you add the next. It must set i.e. go hard to provide a stable base (try not to confuse 'set' and 'dry').
I suggest you may be better with the one coat for the crack you describe, rather than render.

Normally a render would have a scratch coat put on, scratches put in and left to set for few hours (maybe even next day) and then put on the float coat over the top. It's important to note that one-coat or other gypsum plasters behave differently to render.
Plasters tend to set quickly after an hour or so of adding water to mix. Cement has a different reaction and 'sets' over a longer period. When applying multiple layers, this longer delay in the setting could be a hassle.

You should be able to complete the repair in one day with one-coat plaster and put your mesh into the first coat. It will have set by the end of the day, and the dry out will happen over the next week. Then you're good to paint or paper.

Yes, I would use diluted PVA to control suction of the surrounding substrate before starting.

I'd steer clear of DIY deep hole fillers - use a plaster (even one coat variety).

 

[randers]

Thanks for that DIYn99 - very clear for a simple DIYer like me to follow.

I excavated the hole...and found that it looks like an old repair where a couple of half-bricks had been used in the internal block part of the wall, and the cracks had formed around their movement. (I've cleaned it up, treated with PVA and will make a bricklaying mortar to repair it properly).
Once that's done it won't be a deep fill any more - so sorry but it's another variation on the same question again! - how long before I can put a one coat plaster on top of a new couple of bricks & mortar? Thanks loads - and sorry to be a pain!

 

[NickB_99]

Thanks for that DIYn99 - very clear for a simple DIYer like me to follow.

Started there myself. Still a diyer, but less newbee than I was!

- how long before I can put a one coat plaster on top of a new couple of bricks & mortar? Thanks loads - and sorry to be a pain!

If you put some mortar in there, as a diyer myself, I'd probably leave it then to set overnight. Wet it all before you start to mortar anything though.
Next day I would probably be over cautious, but I'd use some diluted pva before putting the first plaster coat in. Then repair as discussed.

Pros are more conscious of time, so they may have tips to complete the repair in a single day.