Furniture Detective: Patch up your respect for Bondo


Very often in the course of restoring a piece of antique furniture it is necessary to fill a nail hole, plug a crack, fill a divot or even replace a missing area of trim. For this purpose there is a variety of generically called “wood putty” or “wood dough” products available commercially plus the never ending list of custom recipes which may include such esoterica as coffee grounds and floor sweepings mixed with glue and linseed oil. One thing they all have in common is their inability to accept a stain just like the surrounding wood. All the products will claim to be stainable and that’s true but they don’t stain like wood. True, some are better than others and some even come in colors but get over the notion that a filler will look like wood without further touch up and the touch up is a subject for another day.

Types of fillers

Fillers come in three main types: water based such as Elmer’s Putty, solvent based such as Famowood and Fix Wood Patch and catalytic such as Bondo the auto body filler and a Minwax product of similar make-up. The water based putty is easy to use on very small areas such as nail holes and is safe and non-toxic. On the other hand it shrinks a lot and has to dry overnight. The solvent based products are much faster drying and can be sanded in a matter of 20 minutes or so. They do exhibit some shrinkage but not as much as the water based putty. Being solvent based they do require a little more care in handling and application but the results are usually worth additional efforts and working time is greatly enhanced. But even the solvent based material is not really strong and cannot be used to recreate missing pieces. The real major leaguer in the filler family is the catalytic product.

This series of products gets its speed and strength from the mixing of a hardener with the main ingredient. The result is a pasty substance that generates heat as the chemistry combines the ingredients and the mixed portion dries very fast and very hard. It can be sanded, sawed and shaped after drying into almost anything given proper preparation but in the end even this product, like all of the fillers, is not structural, will not carry much weight and will not reliably hold a nail or screw for any length of time or stand a great deal of stress. What it will do is provide a stable background for you to demonstrate your artistic ability in simulating wood.

Use of the catalytics is called for when time is of the essence, when the areas to be filled are larger than a screw hole or when the filler will need to be carved or otherwise manipulated to match the background.

Preparation

Preparation of the wood prior to using the filler is critical. The wood must be dry without traces of stain or stripper in the area. The area to be filled should be dug out using a razor knife or similar tool to expose new wood on all sides of the defect, eliminating any trace of dirt and chemicals lodged in the cavity. The edges should be irregular to provide the filler a good surface to grip so it won’t pull out when sanded.

For larger applications grooves cut in the wood help hold the filler. If the application is going to be free standing when it is finished, such as a corner or trim piece, give it some help by constructing a support structure before applying the filler. Do this by installing a series of very small finishing nails throughout the area to be filled, driving the nails in at odd angles with their heads just below the anticipated level of the finish, thus giving the filler a web to cling to as it dries. Staples installed in the work area also make a good matrix for holding the filler. Any of the metal that ends up protruding too high can be recessed with a center punch and patched later.

As the final step in preparation, apply masking tape along the outside edge of the defect all the way around. This will do two things: It will protect the surrounding area of good wood when you sand the filler and it will keep the filler out of the open grain of wood such as oak and mahogany and make your touch up easier later.

Application

Using a 1-inch putty knife mix the body of the filler with the hardener on a piece of stiff cardboard that can be discarded later. Most mixing directions call for a “BB to a golf ball” ratio of hardener to body but try using half that amount to a BB. It sets up quicker. Mix the filler vigorously until the color is uniform and apply to the wood without delay. Use only the putty knife. Do not use your hand because it may distress your skin and it will stick to you. Apply the fill in layers in the defect and make sure all the areas are filled looking carefully for bubbles and voids. Overfill the area above surface level to allow or minimal shrinkage and bear in mind that many projects will require two or three applications to get it right. You can pile up the filler on edges as it starts to stiffen slightly while you work and you can “tease out” corners to shape before the filler sets up.

Shaping

Within 5 to 10 minutes of application the mixture will be set up enough to shave it down with a break-off-blade knife to basic shape, eliminating much of the later sanding of a rock-hard mixture. Take care that you do not pull or stress the patch too much. While it is firm enough to hold its shape it is not set up fully yet and is not holding all that well. When the mixture has fully cured, 20 to 30 minutes, it can be sanded by block or shaped with a rasp for another application.

Caution

The catalytic reaction used to produce such a filler medium also produces fumes and some people are very sensitive to them. Also the mixture does generate varying amounts of heat as a by-product and the uncured mixture may get uncomfortably warm to the touch. A dust mask should always be worn when sanding these catalytic fillers. The dust is exceptionally fine and may cause respiratory irritation.

In other words, use your head when using these as well as all other finishing materials and products. ?

Send your comments, questions and pictures to Fred Taylor, P.O. Box 215, Crystal River, FL 34423 or info@furnituredetective.com. Visit Fred’s Web site: www.furnituredetective.com.

His book “How To Be a Furniture Detective” is available for $18.95 plus $3 S&H. Also available is Fred and Gail Taylor’s DVD, “Identification of Older & Antique Furniture,” ($17 + $3 S&H) and a bound compilation of the first 60 columns of “Common Sense Antiques by Fred Taylor” ($25 + $3 S&H). For more information call 800-387-6377, fax 352-563-2916 or info@furnituredetective.com.


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