Which fill-in methods make the most sense? - Woodshop News

Which fill-in methods make the most sense?

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Today, most finishers use wax filler sticks that can be melted and dripped into a void. For small defects, such as open joints and scratches, the filler sticks can be rubbed vigorously into the void before leveling. While the wax formulations have the least durability of the fillers we’ll explore, some of the harder versions are solid enough to serve in relatively low-wear areas if they are coated with a durable topcoat.

Shellac stick patching refers to any of a number of thermoplastic resins used in producing what are generically called “burn-in sticks,” even though there are few burn-in sticks that still contain shellac, which has largely been supplanted by nitrocellulose, acrylic and epoxy resins.

Acrylic resins make for a good non-yellowing, water-clear stick, but are softened by lacquer thinner. As a result, a repair will “gloss up” when topcoated with lacquer.

Thermoplastic epoxy resins offer better stability and solvent resistance and are the top choice for today’s burn-in sticks.

Burn-in basics

While there are many different procedures the finisher can use to apply, level and smooth burn-in sticks, the basics involve using a heated metal blade to melt the resin and overfill the void. It might be necessary to gently stir the molten material so it contacts the void completely. As the material cools, wet your finger with water and flatten the fill to penetrate any damaged wood fiber and release any trapped air.

With a burn-in knife, you can use a lubricant applied over and around the filled void to complete a surface fill. By holding the tool at a low angle, pull the edge of the blade in a smooth and unbroken motion. Repeat until the material is flush or very slightly above the surface. The lubricant won’t prevent a hot blade from damaging the finish, but it will help the blade slide smoothly over the surface and prevent the burn-in material from sticking to the surface.

Once the burn-in material has completely cooled, it can be sanded flush or rubbed with a felt block with a solvent that will affect the burn-in stick, but not the surface of the object. For instance, an epoxy-based burn-in stick can be used on a high-pressure decorative laminate and leveled with a block wetted with lacquer thinner. A shellac burn-in stick on a conversion varnish can be leveled with a block wetted with denatured alcohol.

Colored or clear hot-melt adhesives are also used to fill voids and are typically tougher and more flexible than the harder resins listed previously. They can be leveled with a chisel or a specialty tool designed to cut the fill flush with the surface.

Two-part clear fillers

Two-part epoxy resins can be used to make clear fills, but be aware that some will yellow with age.

To use these on a finished object, mix the resin with the catalyst or hardener very thoroughly and drip or pour the epoxy into the void. Since there will be very little shrinkage, it is not necessary to build the epoxy proud of the surface, especially in shallow voids. A caul of polypropylene or polyethylene plastic works well to hold the epoxy in the void. If the caul is cut slightly larger than the periphery of the void and smoothed on with a credit card or pallet knife, most of the excess fill can be pulled away from the void and cleaned off the surface with alcohol. This helps to minimize the sanding necessary to level the fill.

Applying gentle heat with a hair dryer or heat gun will lower the viscosity of the epoxy, making it flow better, speed up the cure and help remove any bubbles in the mix. Epoxy resin can be colored with dry pigments and compatible pigment stains, mixed with micro balloons, bronzing powders, sawdust, silica and other particulate matter to match many surfaces.

Single-component, ultraviolet curing epoxies are also available. Some will cure well in sunlight, while others might need a more powerful UV source.

Epoxies tend to be tougher and harder than lacquers and varnishes and could be more difficult to sand smooth. But when heat, solvent, scratch and impact resistance are important, they can be very useful.

Polyester resins can also be used for clear or colored fills. They behave much like epoxies, but typically have a somewhat short shelf life and an objectionable odor.

Cyanoacrylate adhesives

Cyanoacrylate adhesives have received mixed reviews since they were introduced to the woodworking world in the 1970s, largely due to their misuse and poorly understood characteristics. Their effectiveness as a fill is due to their solvent-free nature, fast cure time, water clarity and ability to be polished to a high gloss.

The viscosity can vary from water thin to a paste. The water-thin CA can be used to reconsolidate a fractured substrate, such as with fiberboard or particleboard. It can also consolidate fractured coatings. For hairline cracks on a thick, brittle coating like UV-cured polyurethane or polyester, thinned CA can be wicked or forced into a crack.

While there are many different formulations of CA for different uses, most cure when pressed into a thin film in the presence of surface or atmospheric moisture. Higher viscosity CAs can benefit from the use of an accelerator or activator, typically an alkaline liquid, sprayed on one of the surfaces to be bonded, especially when the glue line is not a tight fit.

Because most CAs cure quite hard, it is difficult to sand down a high fill without marring the finish. There are a couple of tricks that make this less of a problem. One is to use a polypropylene caul as described earlier for an epoxy fill. Another is to fill the void as nearly as possible to flush with the surface and then surround the area with a thin tough plastic, such as Mylar shipping tape, to prevent removing too much of the original coating when sanding. For fills on surfaces that are not affected by the solvents used in debonders (acetone, nitromethane, methylene chloride), rub the fill with a hard felt block dampened with the solvent or use the solvent as a lubricant for fine sandpaper.

This article originally appeared in the April 2016 issue.

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