Hello again, today I would like to talk to you about an unpleasant aspect of surface preparation, dents and dings. Although we would like to handle material completely by machines with no worries about mishandling the fact is we can’t. Woodworking materials are handled several times by humans. This means that the materials will be dropped, bumped, scraped and scratched along with several other forms of accidental abuse. Some of the damage caused by mishandling will be too deep to be removed by sanding. In other words, as long as wood is handled by people, there will be holes that will need to be filled. I am a firm believer in the old metaphor “there is no hole too big, just a can of filler too small”. Although I believe in that saying, I do realize that certain fillers are needed for certain types of damage. I will explain the different types of fillers available for wood according to the depth of the repair and the time needed for it to be done. I will also focus on repairing damage for projects that will be finished in a transparent or stained finish because these are the hardest to repair. All of the techniques mentioned below can be used for opaque finishes without having to be concerned with color matching. Let’s get started.
I would like to start by addressing repair of large, deep holes, dented edges and broken corners. These types of damage can be created by dropping wood on its corner or edge or by dropping a sharp, heavy object in the middle of the board. I will classify this type of damage as deep damage, having a depression that is ¼” deep or more. The first product that can be considered is the time tested classic: Bondo. Bondo is a trade name for polyester based automotive body filler. Most of you know about these products but I want to mention a few do’s and don’ts that you might not be aware of. First of all, automotive body fillers use cream hardeners usually colored red or blue. When mixed, these products leave a pink or aqua colored fill that is difficult to cover when doing a stained finish. For clear finishes it is out of the question. One coatings supplier that I am aware of makes a cream hardener that is light beige making the fill closer to a wood color and easier to touch up when staining the wood. The most important thing to remember when mixing body filler is DO NOT OVER CATALYZE THE FILLER. Contrary to common believe, body filler will shrink back into the depression one or two days after being catalyzed if too much hardener is added. This happens because, when catalyzed correctly, body filler is nearly 100% solids. Cream hardener lowers the solid content slightly. When too much hardener is added it lowers the solid content enough to shrink into the hole. Your perfectly prepared fill now shows up in the finish leaving a noticeable depression. Follow the directions on the can which usually recommend taking a 3” diameter disc of filler and adding a three inch strip of hardener. A little less hardener will still cure completely. A little too much will become an issue. Also, when applying water base finishes, complete curing of the filler is essential. Uncured or “green” filler will react with most water borne finishes causing them to alligator or crawl off of the filler.
Another product that is absolutely essential in your arsenal of dent repair products is an epoxy filler known as an epoxy stick. This product is a two component epoxy in a pliable stick form. It has the hardener component inserted into the middle of the stick. When a short length is removed and kneaded together with your fingers it mixes the hardener with the resin starting the curing process. The open time for using the epoxy is five or six minutes. This product is exceptional for filling large dents, large holes and rebuilding destroyed corners. It cures to a very hard material with excellent strength and adhesion to the substrate. When hard it can be screwed, nailed or cut to size. The popular touch up supply companies provides these epoxy sticks in several colors. This gives you the ability to make a repair that is close in color to the finished product, making the touch up process easier. The last product I would like to discuss for deep repairs is little known in the United States and, in my opinion, one of the best solutions for large repairs. It goes by the trade name “knot filler”. It is a complete line of polyamide based hot melts sticks that are available in multiple colors and three levels of flexibility. The level of flexibility needed depends on the products to be filled. It provides a reliable, non-shrinking fill in less than 30 seconds. I do not make it a habit of recommending suppliers in my articles but I know of only one supplier of this product in the United States.
Rangate Tool in California is the only supplier of this product to date. Please visit www.rangate.com for a comprehensive presentation of this innovative new filler system. This is all of the room I have for now. Next time we will discuss the filling of shallow holes and dents along with methods of touching up the repairs. Until then, keep finishing.