Processes Used to Finish Wood Materials Chapter 36 Processes Used to Finish Wood Materials

Objectives Preparing the surface before applying the coating. Filler to close the pores before applying the final finish for open grain woods. Electrostatic spray painting finishing process for furniture.

Surface Preparation Not all finishes work equally well on all woods. Sanding: It is the first step in surface preparation. Material removal by use of abrasives is still called sanding. Roughing is done with a coarse abrasive to remove stock. Blending is done with a medium grit abrasive to produce a smooth finish. Fine finishing involves using fine grit abrasive to remove light scratches.

Applying Fillers With open grained woods such as walnut, oak, mahogany, or ash, a filler should be used to close the pores before any finish is applied. Paste fillers contain powdered quartz, linseed oil, turpentine, and drying agents. Before a wood is filled, you must first clean the pores with a wash coat of shellac or lacquer. The first coat is applied in the direction of the grain. The second coat is applied against the grain to ensure complete coverage. Wiping excess material should be done while wet with a rag with the grain. Once the filler has dried, the wood is ready for final finish with stain, varnish, paint, enamel, or lacquer.

Bleaching Wood Bleaching is done to lighten the natural wood coloring. In large volume industrial applications, the bleach is typically applied by spraying it onto the wood surface. Bleaching removes color by oxidation. It is difficulty to drastically lighten darker woods. Commercial bleaches usually consist of hydrogen peroxide and caustic soda. Whenever bleaches are used rubber gloves and eye protection should always be worn. Once the desired degree of bleaching has taken place, lightly sponge the stock with warm water to neutralize (stop the action of) the bleach and remove any residue. Water is often applied to both faces of the wood to equalize the moisture, so the wood does not cup (assume a concave shape).

Shading Wood Shading or glazing is a process that is sometimes used to improve the color uniformity of the wood surface. Shading differs from bleaching because it is carried out after the wood has been sealed with filler. The process involves spraying color tinted shading lacquer over the area to be shaded.

Staining Wood Stain is used to change the overall wood color or to beautify the grain by either adding emphasis or lightening the grain pattern. There are four basic types of wood stain: water stain, oil stain, non grain raising (NGR) stain, and spirit stain. The major disadvantage of water stain is that it will raise the grain in the wood. Non grain raising (NGR) stains are widely used by furniture manufacturers. They have a short drying time. Spirit stains are used mostly for repair or touch up work. Once a stain is applied and allowed to dry, it should be sealed with shellac or lacquer based sanding sealer. The sealer protects the finished surface.

Finish Coatings Finish coatings are usually a clear coating of oil varnish, lacquer, or synthetic resin varnish. Varnishes and lacquers: A varnish topcoat is applied to protect the surface from water spotting, chipping, and abrasion. Lacquer is highly volatile and the fumes are toxic. It should be handled and applied with care, making sure there is adequate ventilation. Paints and Enamels: Paints and enamels are opaque finishes that hide the grains of the wood. Latex paint cannot be applied over oil based paint as the finish will bubble and peel. Enamel has a gloss finish and is more durable than latex paint.

Application Methods Atomized spraying: In atomized spraying, pressure is used both to break the finishing material into very fine drops (atomize) and to blow it onto the item to be coated. After the product is sprayed, the finishing material is either air dried or heat dried. Robotic painters are used in many companies to ensure a consistent paint thickness, reduce spray time, and remove workers from the spray environment. Electrostatic spraying: In electrostatic spraying, the finishing material is fed to a specially designed spray gun, where it receives a negative charge. The product to be coated is given a positive charge. Even thickness can be applied with little overspray and wasted material. Electrodeposition: The part to be coated is placed in a vat of special paint and then attached to the positive side of a charging circuit. The paint particles contain a negative charge and are attracted to the positive anode. After the product is coated it is placed in an oven for drying and curing. Roller coating: The process consists of squeezing the stock to be coated through rubber rollers carrying a film of the finishing material. In reverse roller process the rollers turn in one direction and the stock moves in the other. The result is better coverage of the material on the work piece. Dip coating: The process is practical for coating parts, like auto bodies that are difficult to finish by any other method. Tumbling: Can be used to finish wood knobs, golf tees, beads and other small parts. The objects to be finished are placed in a drum or barrel with a small amount of lacquer, enamel, or wax. Forced drying: Parts are loaded onto conveyors that carry them through heated ovens. Forced drying is often done with ultraviolet (UV) radiation. The finish coat is cured by light rays, rather than heat.

Summary Sanding: It is the first step in surface preparation. Material removal by use of abrasives is still called sanding. With open grained woods such as walnut, oak, mahogany, or ash, a filler should be used to close the pores before any finish is applied. Bleaching is done to lighten the natural wood coloring. Shading or glazing is a process that is sometimes used to improve the color uniformity of the wood surface. Stain is used to change the overall wood color or to beautify the grain by either adding emphasis or lightening the grain pattern. Finish coatings are usually a clear coating of oil varnish, lacquer, or synthetic resin varnish. A varnish topcoat is applied to protect the surface from water spotting, chipping, and abrasion. Paints and Enamels: Paints and enamels are opaque finishes that hide the grains of the wood. In atomized spraying, pressure is used both to break the finishing material into very fine drops (atomize) and to blow it onto the item to be coated. Forced drying: Parts are loaded onto conveyors that carry them through heated ovens. Forced drying is often done with ultraviolet (UV) radiation. The finish coat is cured by light rays, rather than heat.

Home Work 1. Explain the 3 basic types of sanding? 2. What is the advantage of a varnish topcoat? 3. What is the advantage of dip coating?