How to install cornice like a professional

Some people think that installing installing cornice in their home, especially MDF cornice, is too difficult and time-consuming for an amateur renovator and should only be done by professionals. Others think it is simple and easy and anyone can do it quickly in only about half an hour. In truth, almost any person with basic carpentry skills can do it; but to do it right, you must work carefully to avoid mistakes and take time required to do it right.

This brief tutorial is designed for the first time installer. It is the same system for installing MDF used by professional installers but explained in a very simple way so that the average home owner can easily learn and understand. Any terminology or special tools required will be explained and described thoroughly so that you can finish your work just like the pros.

Site Preparation

This project requires you to do sanding and will produce a very fine airborne MDF dust. This dust will eventually settle so you might want to cover the furniture to minimize the amount of time required for cleanup when you are done. You will need dust masks, and a vacuum and rags to wipe dust away. The walls should not need to be covered as is required when dealing with plaster dust. You can do all of your cutting of cornice material outside of the house to lower the amount of dust you generate. You should use the vacuum regularly to keep dust levels to a minimum.

Tools Required

It is always a good idea to have the tools you need to finish the job ready before you need them. So we made a simple list of what you will require to do a professional job on this installation. You may already own some of the tools required and you can rent the others for a few days.

1. Compound Miter Saw (10″ draw)
2. Professional measuring tape
3. Digital Angle Finder
4. Finishing Nail Gun (Air or electric)
5. Vacuum cleaner & soft dust rags
6. Brad & Pin Nails (15 to 18 gauge; 1 1/2″ to 2″)
7. MDF Glue (not wood glue)
8. Caulking Gun
9. Caulking Compound (DAP)
10. Spackling Compound
11. Fine Sandpaper (200 grit)
12. Medium Sandpaper (80 Grit)
13. Disposable dust masks
14. Square, flat blocks of wood (to use as temporary joiners)

Many professionals use a pneumatic finishing nail gun and compressor to do their trim installations. If you are reniting the equipment, it may save you money to use a cordless electric finishing nail gun instead. Make certain the gun can countersink the nails slightly below the surface of the trim.

Site Setup

You will need a large open area to cut the pieces of MDF cornice to size. The advantage of doing this in the garage is that the materials can also be stored there before the project. Laying them flat on the floor where they cannot be damaged or scratched. There are many advantages to cutting the cornice on the floor rather than on a table. The most important one is that the cornice can be easily be held securely to get an accurate cut using the compound miter saw. Furthermore, you will need lots of room to move around without the worrying about hitting things. The pieces of MDF cornice can be up to 16 feet long, so find a place with enough clearance to move around safely. Tools can also be stored in the garage and any dust created by cutting is not normally a problem.

Place the Compound Miter saw on the floor near an electrical outlet and in a safe place. You should be able to make a 10″ cut with the saw in one single, steady pull. You will also need to know how to setup the saw to make a bevel and a miter cut at the same time. If the saw does not come with instructions to show you how, you should ask the person who rented you the equipment to show you how to do it. The Cornice you are cutting can be up to 8″ wide. It needs to have two cuts because it is installed onto the wall or the ceiling at either a 45 or 52 degree angle. Baseboards are much easier to install because they are installed flat onto the wall and and require only a single angle (miter) cut with a saw.

Plan what lengths or Cornice you will need by using your measuring tape to determine the size of the room. Measure all the walls that are to be finished as accurately as possible down to the closest 1/16th of an inch. Make a simple drawing indicating which will require INSIDE corner cuts and which will require OUTSIDE corner cuts.

Measuring and Cutting

Getting precise cuts in our cornice is essential to ensure the material is installed professionally. To do this, we will need to use a angle finder to accurately determine the angles of our walls. Whatever brand or type of angle finders you use, you must be certain it is accurate. Inexpensive ones may not produce accurate results. Never assume that a wall corner that looks like it is 90 degrees really is. It may be off slightly. Once the angle has been measured, use a cutting chart to set the compound miter saw to be certain you get a good fit. Use the 80 grit sandpaper if necessary to make certain the pieces fit perfectly together. If you are not familiar with using the saw we suggest you use a few short sample cornice pieces to make some practice cuts. Once you are able to create good joints using the test pieces you can confidently make cuts on the full length cornice that will be permanently installed.

Fitting and Installing

You should be able to imagine how the joint for each piece of cornice will look once it is installed. Begin to pin items into place once you have cut a few actual lengths on the saw. We suggest you use either an electric or pneumatic finishing nail gun to install the cornice. Never use a hammer to install trim; you risk accidentally damaging the trim if you miss the nail. It is also almost impossible to hold the trim while using the hammer. The nail gun must be set so that it sinks the head of the finishing nail slightly below the surface of the trim. We suggest you apply at least two nails on the top and bottom edges of the cornice every 10 to 16 inches using an “V” shaped pattern. The “V” pattern works well because it will hold the trim securely even if the nails are only attached to drywall and not into a wood wall joist.

How and where you place the nails on the cornice is very important if you want to produce professional results. The flat surfaces on the back of the cornice are designed to fit tight against the surface of the wall and ceiling. These surfaces are where you should place the nails along the entire length of the cornice.  The heads of all the the nails should be slightly below the surface so that is can be filled and hidden with spackling compound later.

All corners should be glued together using special MDF glue. DO NOT use traditional white or yellow wood glue because they do not cure hard and remain slightly flexible even once they have completely dried. Over time, traditional glue will allow the material to slip apart slightly. This phenomenon is called “creep” by manufacturers of MDF products. Glue engineered for MDF cures rock hard and bonds chemically with the material to prevent “creep” that could cause unsightly gaps to appear in the material. Another problem with using traditional glue is that because it is water-based, it will cause swelling of the joints. MDF glue is a chemical formulation that contains water so that it will create almost invisible seams.

Attaching two straight pieces of cornice together requires a different technique than when installing joints. Joints in cornice become necessary whenever you have a straight wall that is longer than any of the the available lengths of material. To make sure you produce professional quality results, you need to take the following steps so that the joint is hidden and strong enough that is will not come apart:

• If you need to use two full lengths of cornice on a wall, avoid joining the two pieces exactly the same length in the middle. Peoples eyes are always drawn to the middle of a room. If there are any imperfections in the joint they will be easily noticed. For example, if you had a wall 20 feet long, don’t use two 10 foot lengths. Use one 12 foot piece and another 8 foot piece so that the joint is not in the middle.
• Do not use a 90 degree cut to join the two pieces together. Cut them at a 30 degree angle to ensure a tight fit and a larger gluing surface.
• Use a 3/4″ thick block of wood to joint the two pieces together on the back side of the cornice. This block will be hidden. There should be approximately four inches on each side which will be nailed. Glue both the block and the joint together at the same time.
• Once it has been glued and nailed, you do not need to wait for it to dry. You can install it immediately into place on the wall

Caulking and Spackle

After you have installed all of the cornice into place, the next step is to use caulking to seal any gaps that appear between the wall, the ceiling, and the cornice. The caulking may sink inwards a little once it dries. However, you SHOULD NOT use same caulking material to finish the surface of the cornice. Use spackling compound or putty to fill all the holes, corners and joint seams. This material is a material designed to be applied to the cornice to hide surface imperfections. Allow to dry completely before it you attempt to sand it using a fine, 200 grit paper. It should be applied liberally so that there is enough material to sand the surfaces smooth. Use a wet sponge to wipe your finger off when you are applying putty to the cornice to fill the nail holes.

Painting and Final Finishing

Once the cornice is installed and sanded it is ready to receive the final finish of paint. This is why you should not paint cornice before it is installed; you will only need to paint it again once it has been sanded smooth. The cornice can now be painted any color you choose using any type of latex or oil based paint. If you are trying to re-create the look of plaster, you should use a flat white paint that will hide imperfections. Remember, glossy finishes will reveal imperfections more than flat or semi-gloss colors.