In situ cornice with bonding & multi?

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Skyshed

Member
Evening all,
I am planning on running a cornice with bonding and finish, did consider adding a drop of lime, could anyone give me an idea of how much lime to use and how long to leave the bonding before going over with multi?
How many metre to run at a time?
The stairwell ceiling in question is about 3.5mx3m so 13m total run, projection about 8", drop about 6",
Useful advice welcome
Cheers
 

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Skyshed

Member
I have run a cornice this way a few years ago, way of suction on bonding, have read that some folk aff lime, to speed setting I suppose?
I fix a6" x2" board, covered in mesh, to studs and ceiling joists to save on core weight, time and materials...
 

BigK

Private Member
If you insist on doing it yourself I'd put in about 1/4 lime. Wouldn't be doing it with multi myself, but there will be people more qualified than me along soon I'm sure
 

tam

Active Member
I think as malc says buying a cornice is the best thing to do

But if you really want to run it you need a good horse(mould with a good steel plate) and your wall and ceiling to be nice and straight. Do 1 side at a time the full length of the wall. Build out with bonding, all but maybe 10mm scratch the bonding to make sure there is a good key. Make sure your horse is running smooth with no heights in the bonding.

Let the bonding set then mix stiff multi, put it on and run the horse on it again, repeat this a few time using the stiff multi and excess that comes of the horse.(filing and running)

When you you feel it is taking shape do a new mix of multi but looser than the previous mix and repeat it again, filling out and running the horse.

Keep doing this until you are happy with the mould. The last mix will be looser than you would skim with and is put on with a brush. It could take up to 4 or 5 mixes in total.
 

Skyshed

Member
If you insist on doing it yourself I'd put in about 1/4 lime. Wouldn't be doing it with multi myself, but there will be people more qualified than me along soon I'm sure
Thanks for the advice.
Can I ask you why you add lime?
 

Skyshed

Member
I think as malc says buying a cornice is the best thing to do

But if you really want to run it you need a good horse(mould with a good steel plate) and your wall and ceiling to be nice and straight. Do 1 side at a time the full length of the wall. Build out with bonding, all but maybe 10mm scratch the bonding to make sure there is a good key. Make sure your horse is running smooth with no heights in the bonding.

Let the bonding set then mix stiff multi, put it on and run the horse on it again, repeat this a few time using the stiff multi and excess that comes of the horse.(filing and running)

When you you feel it is taking shape do a new mix of multi but looser than the previous mix and repeat it again, filling out and running the horse.

Keep doing this until you are happy with the mould. The last mix will be looser than you would skim with and is put on with a brush. It could take up to 4 or 5 mixes in total.
Thanks Tam.
I like the idea of brushing on the final 'slip'..
The walls are surprisingly straight, checked em with 8ft feather edge and you would struggle to fit a rizla down the back of it..
The ceiling was Ali over the place though, I stripped the lime off, left the lath, will prob sheet over it and straighten ceiling out dead flat in bonding.
I like to trowel in a dead tight but of finish with a touch of lime on the 'screed rail' so the surface is nice n hard for running the horse over it.
I could knock it up on a bench but for this short run...
Plus I just fancy doing it in situ for the hell of it !
 

BigK

Private Member
Lime not required if your using bonding & multi. It's used on a different background to weaken the finish layer, and of course it's nice & fine, I'd maybe use some in the wet final mixes, but hey that's just me [emoji51]
 

tam

Active Member
Thanks Tam.
I like the idea of brushing on the final 'slip'..
The walls are surprisingly straight, checked em with 8ft feather edge and you would struggle to fit a rizla down the back of it..
The ceiling was Ali over the place though, I stripped the lime off, left the lath, will prob sheet over it and straighten ceiling out dead flat in bonding.
I like to trowel in a dead tight but of finish with a touch of lime on the 'screed rail' so the surface is nice n hard for running the horse over it.
I could knock it up on a bench but for this short run...
Plus I just fancy doing it in situ for the hell of it !

Yea that's a good idea on the screed rail but I wouldn't bother adding any lime either. I don't know why I just never have so wouldn't chance it.
 

PlasterCraftDundee

Well-Known Member
If you are going to use bonding I would muffle the mould that way the bonding is nice and even right the way through the run. Although if you're building down with plasterboard will you need to bond it out?
 

Skyshed

Member
If you are going to use bonding I would muffle the mould that way the bonding is nice and even right the way through the run. Although if you're building down with plasterboard will you need to bond it out?
Yep I was planning on a muffle/ baffle on the mould, allowing 5-10 mm for to coat.
Only concern really is suction on the bonding for running a cornice...
Should be fine if I get multi going soon as bonding is set?
 

tam

Active Member
Yep I was planning on a muffle/ baffle on the mould, allowing 5-10 mm for to coat.
Only concern really is suction on the bonding for running a cornice...
Should be fine if I get multi going soon as bonding is set?

Yes if you are working on the bonding the same day It should be no problem.
 

Skyshed

Member
Lime stops it expanding , we would run in casting and lime though , so maybe multi doesn't expand as much







Plasterwork. Ie

Ah right, that's handy to know, thanks you
lime also slows the set --- any reason why you re not using casting plaster & lime putty, its a lot finer than multi resulting in a smoother finish.
I'm right up in the north of Scotland so it's difficult to get hold of casting plaster or lime putty at short notice..
Also, the nearest fibrous plaster workshop is over 200 miles away in Glasgow, hence the decision to run in situ.
What mix of lime putty and casting plaster would you recommend? (For future reference)
I usually use lime to make multi/ bonding set faster.
On a restoration job once we used a mix of gypsum and line as a finish for lime on lath but had to use size/ retarder made from rabbit skins as per Historic Scotland specifications.
I don't understand how lime would slow the set of gypsum such as multi?
I am interested to hear about using a mix of lime and casting though..
 

puddove

Private Member
Ah right, that's handy to know, thanks you

I'm right up in the north of Scotland so it's difficult to get hold of casting plaster or lime putty at short notice..
Also, the nearest fibrous plaster workshop is over 200 miles away in Glasgow, hence the decision to run in situ.
What mix of lime putty and casting plaster would you recommend? (For future reference)
I usually use lime to make multi/ bonding set faster.
On a restoration job once we used a mix of gypsum and line as a finish for lime on lath but had to use size/ retarder made from rabbit skins as per Historic Scotland specifications.
I don't understand how lime would slow the set of gypsum such as multi?
I am interested to hear about using a mix of lime and casting though..

hi skyshed

half fill bucket /container with water add the hydrated lime and leave soaking over night - (lime putty )
stir /mix.
pour /feed onto spot board - for a circle/well in the middle add water - then sprinkle casting plaster into the water to soak up the water allow to settle prior to mixing with your handboard /hawk and trowel - should be a stiffish creamy consistency (not stronger than 50/50 ) this is then applied to the coring out (rough shape of cornice formed by running mould & muffle) then pass over with running mould, this process is repeated until the desired finish ( the mix should be slightly weaker to finish off.
i wouldn't recommend using a retarder when running in-situ can cause the mix to become brittle- also tends to ball up when running (y)
good luck with your running -
plenty of photos of the end product :rock:
 
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