There are many styles of trim in materials such as wood, metal, and plastic. Trim is used to cover gaps at inside corners, floor and ceiling joints, or around door and window openings. Trim is the key to a successful job. With trim you cover your mistakes and thereby avoid the need for complete accuracy.
Most Common Mistake
Leaving more than a 1/4" gap at corners.
Floor and ceiling joints
The basic trim piece for a floor-to-wall or ceiling-to-wall connection is a quarter round or baseboard. It offers a clean, straight appearance and is easy to install. Simply nail it in place with finishing nails.
Place your paneling so that the edges meet as tightly as possible at an outside corner. If the intersection of the two pieces is neat enough it can be left uncovered. Special outside corner molding can also be applied.
If your walls are straight enough, and your paneling job neat enough, you can sometimes run one sheet tightly into the corner and butt the mate right up to it for a snug fit and a good-looking job. If this is not the case, however, a quarter round will work nicely here as well.
Outside baseboard corners
These pieces should first be cut to a 45-degree angle with your miter box and back saw. Nail them into place so they are snug. Then run the round part of your hammer handle or screwdriver shank tightly up the joint to seal any gap that may be left.
Inside baseboard corners
These can be tricky when you are using the types of baseboards that have curves and shapes to them. I do not recommend using a miter cut here because the nails have a tendency to pull the intersecting pieces away from each other, creating quite a gap, when attached. If you are painting, this crack can be filled with caulk and painted over. If you prefer the natural wood trim, a process called "coping" will give you a beautifully fitted corner every time.
Begin by butting the first piece of molding up to the wall and nailing it in place. The second piece should be placed in your miter box for a 45-degree cut with the inside wood grain showing on the front of the molding. Use a coping saw kept at a 90-degree angle and cut away the wedged portion of the molding, following the outside line exactly. This method transfers the perfect profile to the molding, which will then fit snugly over the mate at the wall.
Joints along the length of the baseboard can be ugly and tend to pull apart if merely butted up against each other. Another trick here is to miter cut the ends at a 45-degree angle so that the two pieces of molding will overlap each other to make a cleaner looking trim. Apply some glue at the miter joint between the two pieces. This will keep the joint tight if the molding shrinks. You can allow the glue to ooze out of the joint and get tacky. Then lightly sand the joint and the glue will mix with the sawdust and conceal the joint.
Tip: Trim is the secret to a successful paneling job. It covers your mistakes and thereby helps you avoid the need for greater accuracy.