# Forum Home Renovation Plastering  Replacing one length of cornice

## iversons13

Hi all, 
I've recently had a go at fixing plasterboard over an exposed brick feature wall - my first attempt at anything plaster related. So far it has gone quite well (thanks largely to the great info on this forum and how2plaster.com), however I'm now at the stage of replacing the length of cornice that I destroyed and need more guidance! 
As you can see in the attachment, the feature wall has one ajoining internal and one external angle. To fix the plasterboard I roughly hacked the old cornice off at 45degree angles. Should I try and cut a new length of cornice to match the existing angles and attempt to patch up the joins, or should I cut the cornice on the ajoining walls (at say point A and B) and use a miter box to create 3 new pieces? 
Seems as though the second option would produce the better result, butgiven I have zero experience I may be overlooking something!  :Wink 1:

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## rrobor

No dont cut. Cornice is cheap but the only way to get it correct is to make a  mitre box.  Now the mitre box must match the size of the cornice. So Glue two pieces of wood together to make an L shape  probably about 50cm long. get a bit of cornice and sit it in  the L so that the height  it rises up is the same as the length it stretches across  the L. cut the base board on that line and glue the second side on. Cut 2 X 45 degree angles  across at a convenient place and that is your mitre box. Cutting simple cornice there are 4 possible angles it can be, 2 cuts concave, 2 cuts convex. Without a box its a mess. As you have an open corner thats an easy one.

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## iversons13

Thanks for the response rrobor. I'm all for redoing the angles and saving myself all the hassle of patching up dodgy joins. Though I'd probably go down the path of buying a mitre box rather than constructing one.  
I guess what I'm confused about is whether I should make a couple of straight cuts along the cornice on the ajoining walls so I can stick the new pieces on as seamlessly as possible. And if so, what's the best method for doing this?   
Unfortunately replacing all of the cornice in the room is out of the question as it's an open plan lounge/dining/kitchen and hallway with 27 walls.

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## rrobor

Im not sure if you can buy a mitre box for that, I aint seen one and any plasterer I know has several, all home made. I suppose you could get a mitre box and put a strap of wood in it to get the correct width as the width is crucial to the final fit. As to cutting . Cornice does not cut well in the middle. Believe me Ive tried. Perhaps a pro may be able to hide a joint in the middle of a run but as an amateur, dont try. If you damaged the origional cut , its easier to fix in the corner or if the length isnt too great, replace and cut clean

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## Naf

Bunnings sell a mitre box for cornice, it suits a few different sizes, can't remember how much I payed but it wasn't very much, they also sell the gyprock mitre master (maybe spelt differently). Starting to sound like an advertisement for them.

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## murray44

Yep, I bought one of those little red plastic mitre cutters (Gyprock brand) from Bunnings. They look flimsy and cheap (they are) but they work a treat.  
Once you patch up any gaps with a bit of top coat it will look fine. Your mitre cuts don't need to be perfect, you'll just have a bit more patching to do. When you are finished you will probably realise you were worrying unecessarily.

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## tobymcclure

i bought a cornice mitre box from bunnings in melton only a week or so ago.. $25 from memory maybe not even that much.. Dont bother trying to make one for $20 who cares... What is the length of the cornice on the left hand side of the diagram to the internal corner ? if your worried about the join just replace the length of cornice as well that way you would have a nice 45 cut to start from. Dont try a stright cut. i failed hahah  does that make sense.. cornice is super cheap and for an extra $20 i would remove the cornice that finishes on the internal corner, use the mitre bx and bam job done easy

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## iversons13

The length of the cornice on the internal is over 5m and looks to have a straight join in it about 1m away from the corner...though you can only just see the join. I presume this is because cornice comes in about 4m lengths.  
The other issue is the cornice on the external is at least 15mm short since the plasterboard brought the finish width of the brick feature wall out that far. That seems like a fair bit of patching if I just put one new piece up.   
Still confused.

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## rrobor

If I were cutting on a length I probably could do a reasonable job by peeling the paper. You will find its quite easy to scribe the top of the paper and peel a layer off. This gives you a chance of adding a new  section ov paper over the joint. so it can be done. But cutting whilst installed, good luck, I wouldnt try.

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## triple b

Gday I m a plasterer
firstly grab a mitre box,even cheaper from plasterers supply store.
Now this all depends on how badly you butchered the internal mitre on the cornice.
Dont worry about the 15mm gap at the external mitre,its filled easily enough..
first cut a small length of cornice about 400mm long with an internal mitre cut on each end,(see rods excellent site).this will be a template.
Then put the template up against the butchery that is the mitre you roughly cut.You will soon see how close your original cut is.
You may be lucky and have to trim some back with a sharp stanley knife.If the gap is  big , you can fill the damage with cornice cement after the cornice is up.it may take a few coats.If the gap is real big cutting the cornice thats on the wall would probably be the go.
Again you can use the template to get this at 45 degrees.
Hope this isnt too confusing,Its much easier to show than to explain .Good luck..
Brendan
Trowel jockey.

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## triple b

One more thing
If the ceiling is painted already ,sponge on some bondcrete and water mix over the paint where the cornice will go,put the cornice up while the bondcrete is still wet ,
This simple trick should stop the cornice from cracking off the ceiling in a weeks time.
hooroo
Brendan
Trowel jockey.

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## iversons13

Great advice, thanks a lot Brendan...I'll see how I go. Trimming the cornice is a bit of a pain in the ass though... it's an old plaster type rather than paper.

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## Rod Dyson

> Gday I m a plasterer
> firstly grab a mitre box,even cheaper from plasterers supply store.
> Now this all depends on how badly you butchered the internal mitre on the cornice.
> Dont worry about the 15mm gap at the external mitre,its filled easily enough..
> first cut a small length of cornice about 400mm long with an internal mitre cut on each end,(see rods excellent site).this will be a template.
> Then put the template up against the butchery that is the mitre you roughly cut.You will soon see how close your original cut is.
> You may be lucky and have to trim some back with a sharp stanley knife.If the gap is big , you can fill the damage with cornice cement after the cornice is up.it may take a few coats.If the gap is real big cutting the cornice thats on the wall would probably be the go.
> Again you can use the template to get this at 45 degrees.
> Hope this isnt too confusing,Its much easier to show than to explain .Good luck..
> ...

   :2thumbsup:

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## triple b

Ok ,try cutting the mitre on the wall with a keyhole saw,the sharper the better.
If its a larger cornice than 90mm it wont fit into a standard mitre box.
You could make a mitre box ,alot of work for 2 cuts.Or you can cut it using a combination square as a guide.Make sure you cut your template using the same cutting method as the main cornice .
Brendan

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## iversons13

Thanks everyone. I ended up buying a mitre box and whipped up a template. After a bit of work with a blade the gap on the internal came up pretty good. I've got about a 20mm gap on the external as I thought.  
What's the best method and compound for filling up a gap of this size? A fast drying solution would be nice..this project is meant to be completely finished by tomorrow night :O

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## rrobor

Cornice adhesive but dont leave it proud. then if you have some,  top coat so you can shape it with a bit of sandpaper

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## Rod Dyson

Use the cornice adhesive.  As it sets you can cut it back (scrape) to a nice sharp finish. Cornice cement can be re worked to soften as it sets to fill in any areas under filled.  If required you can touch up with sand paper directly sanding the cornice cement.  It can even be further scraped back once dry is required (last resort though)

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## tobymcclure

> Thanks everyone. I ended up buying a mitre box and whipped up a template. After a bit of work with a blade the gap on the internal came up pretty good. I've got about a 20mm gap on the external as I thought.  
> What's the best method and compound for filling up a gap of this size? A fast drying solution would be nice..this project is meant to be completely finished by tomorrow night :O

  
cornice cement mix up a small patch..  and in small ammounts fill the hole , add house hold salt to fasten the process..  you will need a small tool , or a really small blade / skraper.. Keep on filling it till your happy, sand it .. job done

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