How to Repair Cornice Mouldings

When it comes to repairing cornice moulding, the time and craftsmanship involved in the process cannot be underestimated. Jason Bushnell owner of Pembrokeshire Limework was kind enough to share his process with us, as he and his colleague Tony Priest worked on repairs for a local manor house.

(Inspirational architechural feature images from Farrow and Ball from left to right: A Modern Twist painted in French Gray No.18 & Mahogany No.36 | Estate Eggshell; London Stone No.6 | Estate Emulsion. Subtle Flourishes painted in Stiffkey Blue No.281 | Estate Emulsion; All White No.2005 | Estate Eggshell. Piano Room painted in Mouse's Back No.40 | Estate Emulsion & Off-White No.3 | Estate Eggshell).

Jason and Tony were tasked with repairing a number of sections of damaged cornice, which in some cases involved making good the surrounding ceilings as well as casting and fitting new sections of cornice. The first sections were relatively plain which meant they could cast them using what’s called a ‘running mould’. The second was a little more intricate and involved the additional casting of detailed enrichments (individually applied elements) which combine to create a more elaborate cornice design.

Step by Step: How to repair Cornice Molding

“To start with we needed create a mould which we could use to cast the new sections of cornice. We began by obtaining a profile of the existing cornice, by cutting a straight line through it and taking out a small section. This gave us a cross section from which to create a template.

"We used the template to make an outline on a piece of zinc plate which we then cut out as accurately as possible and finished off filing by hand until it matched the cornice profile perfectly.

"Next, we cut a second slightly smaller template out of backing plywood and attached it to the zinc plate. (The plywood gives the plate the extra strength it needs during the casting process but leaves the zinc profile proud enough to still create the cornice detail.)

"The whole thing was then attached to a larger section of wood as shown in the photograph below, to make our right angled running mould. 

"Now we could set up the bench mould which would give us our wall and ceiling lines... 

"On a long flat bench, we used lengths of timber to build up raised sections over which the cornice would be cast. (This reduces the depth of plaster required and in doing so, the weight of the finished cornice section.)

"Once complete, we added a coat of shellac to the bench mould to seal all the surfaces, followed by a coat of releasing agent to help when removing the cornice at the end. 

"We then cut a piece of hessian to cover the bench and 2 similarly sized wooden laths. These were placed along the length of the bench and the hessian wrapped around the laths making sure to fully enclose them. (The hessian and laths are included to strengthen the cornice and provide extra support if needed when fixing it in situ). 

"Now we were ready to start running our cornice. 

"We mixed the moulding plaster and began to pour it onto the bench, being careful to thoroughly bed in the hessian and laths.

"We did an initial run with the right angled mould, to make sure it passed through the plaster covered bed without catching on any of the supports. When that ran smoothly, we gradually began to build up the cornice by pouring layers of casting plaster into the bed and pulling the mould through. As we repeated the process, we gradually saw the cornice taking shape until eventually our replica was complete. 

"When the plaster was set, we removed it from the bench and it was now ready for installation. 

"The following photograph shows the area where our new cornice was being installed. The adjacent ceiling had been damaged slightly and so we re-plastered the edges ready for the cornice.

"It was then simply a matter of cutting the new section of cornice to size, attaching it and gently blending in the edges for a seamless fit.”

Step by Step: How to Repair Decorative Plaster Mouldings

As the next photographs show, the second section of damaged cornice was more intricate in design, with extra detailing (known as “enrichments”) which needed to be cast separately.

“Luckily there were enough enrichments left on the damaged section of cornice, from which we could make our moulds.

"We started off by cleaning the layers of old paint off the individual enrichments we’d taken from the original cornice. We did this by soaking them in warm water and for tougher areas, used a small hand steamer to get the last bits of paint off. When thoroughly clean, we stuck the individual enrichments to a bench and made small wooden frames around each. The frames were slightly taller than the enrichments to make sure that the silicone we were using for our moulds, could not escape.

"The silicone mix was then pored over each enrichment, being careful to completely cover each whilst not leaving any air gaps. Once set, we removed the frames and took out the enrichments, to leave our perfectly detailed silicone moulds. 

"In order to make sure that the silicone moulds are supported enough to prevent them from distorting during the casting process, we made a new set of frames for each of moulds. Again we made them taller than silicone. After spraying the inside of each mould with a releasing agent, we poured in casting plaster and inserted a small amount of hessian with the mixture for extra strength.

"This process was repeated until we had enough enrichments to complete the detail on the section of cornice we were replacing.

"Finally we were able to attach our enrichments to the finished cornice and blending them in with the existing cornice".

Jason Bushnell is owner of Pembrokeshire Limework. You can find out more about his work at www.pembrokeshirelimework.co.uk

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