Tuesday, 07 September 2010 22:28
By Jim Hingst
Staining and varnishing is a great way to finish an interior sign, if the wood used has a beautiful natural grain (such as mahogany). Stains contain pigments that tint and accentuate the grain of the wood.
Varnishes generally are some of the best protective finishes for wood. They consist of an oil or water vehicle (also referred to as the carrier) and some type of resin, both of which contribute to the physical properties of the varnish. (Note: Some of the better-known resins are alkyds and polyurethanes.)
Varnishes come in many varieties. Some are oil-based; others are water-based. In the case of oil-based varnishes, the ratio of oils to resin significantly affects the performance characteristics of the varnish.
Painters commonly classify oil-based varnishes as either long oil varnishes or short oil varnishes. The long oil varnishes contain a high ratio of oil to resin. The long oil types are also called “Marine” or “Spar” varnishes. The short oil varnishes, on the other hand, have a much lower ratio. It’s the oil that makes the varnish flexible and less likely to crack. That’s why the long oil finishes are much more durable for exterior applications, because they expand and contact as the wood expands and contracts.
Coatings of stain and varnish usually will not provide long lasting protection for outdoor signs, which are subjected to the bleaching rays of the sun and blistering heat. For extended durability, signs to be installed outside should be painted.
If you do opt to stain and varnish your sign, the two primary considerations are aesthetics and surface protection:
Aesthetics: Select a stain that enhances the natural beauty of the wood’s coloring and grain. Stains are best applied with a rag or cheesecloth.
Surface Protection: In the sign industry, sign makers often choose polyurethane varnishes (such as spar varnish) to protect the wood from potential damage. In general, varnishes provide good protection against solvent spillage, moisture, abrasion and the degrading effects of high summertime temperatures.
Like everything else, oil-based varnishes have their advantages and disadvantages. On the plus side, they provide good protection for indoor applications. They’re also easy to apply. Because varnishes cure at a slow rate, they have time to level out allowing brush marks to disappear.
On the other hand, the extended open time can also be a big disadvantage. Polyurethanes can take as long as twenty-four hours to dry. If you apply multiple coats of varnish, production time can extend to several days.
A bigger problem is dust. The longer the coating stays wet, the more likely it is for the coating to trap dust in its surface. To minimize the amount of dust and debris settling on the surface of the varnish, you should follow these painting practices:
* Keep your work environment clean.
* Don’t clean just prior to finishing. Cleaning will send dust airborne.
* When cleaning, wet mop your floors and dust with a damp sponge to keep dust down.
* Prior to finishing, wipe the substrate down with a tack rag.
* Between coats, wet sand the finish. This will remove any dust that has settled in the finish and keep the level of airborne dust low.
* Cover the varnished piece with a makeshift tent of visqueen or some other plastic film, to prevent contamination.
Polyurethanes produce a very slick surface. To ensure good adhesion of one coat of polyurethane to another, don’t forget to lightly sand between coats. If you decide to use a polyurethane varnish, many of the pros recommend a conventional solvent-based one. These varnishes are tougher, more durable, and clearer than the water-based products.
Compared to a standard polyurethane varnish though, water-based polyurethanes are significantly lower in Volatile Organic Compounds (VOC). Nevertheless, under pressure from governmental requirements and environmental activists, manufacturers of conventional varnishes in general have cleaned up their act. With the use of synthetic resins, many of the oil-based varnishes manufactured today feature improved outdoor durability. Toxic materials, such as lead, have been replaced with safer ingredients.
If you “test, don’t guess,” you’ll discover what works and what doesn’t work. “I always try to push the envelope to the extreme until I find the failure point,” advises Butch “SuperFrog” Anton. “Once that’s established, I can adjust my paint formulations and my techniques.”
Many sign makers will thin the first coat of varnish, so it will sink into the wood. After experimenting with thinned coats of varnish, just follow the manufacturer’s instructions. To prevent any clumps getting in the coating, Dr. Francis Lestingi of Signs of Gold (www.signsofgold.com) also suggests straining the varnish before using it. Then he applies three or four coats.
Try different products on pieces of scrap wood and mount the test samples in a place where they receive the maximum exposure to UV light. The time spent on these experiments can help you select the right product as well as minimizing product failures. Once you find a product that works for you, stick with it.
While staining and varnishing can bring out the beauty of wood and give character to a bland one, the end results can vary. Some woods—such as pine and basswood—often don’t accept stain evenly and can look blotchy. What causes the blotchy appearance is that the stain will soak into one part of the wood more than another. Applying a wood conditioner prior to staining can prevent this from occurring. (Note: In many cases, the company that makes the stain will also make a wood conditioner. Always use products, which are matched for compatibility.)
Before you stain and varnish a production piece, test the finish on a scrap piece of the wood that you are using or test the coating on the backside of the sign. A final coat can sometimes darken a stained piece. Some woods look better unstained. Only stain something if it improves the beauty of the wood.
Here are some tips for staining and varnishing wood that will improve your finishing results:
* If you decide to sand your work, make sure that you sand in one direction with the grain using long strokes.[B\] Sanding against the grains produces unsightly scratch marks, which will be very noticeable, after you stain the wood.
* Always use a sanding block. A block will distribute the sanding pressure evenly, so the surface of the substrate will be even.
* Regularly clean off the debris from the surface being sanded. Particles between the wood and sandpaper can cause scratches in the wood. Use a brush or vacuum for cleaning; then wipe away remaining dust with a tack cloth.
* Easy does it. Use light pressure when sanding. Excessive pressure can scratch the wood.
* Change the sandpaper frequently (as soon as it clogs up).
* Start sanding with a coarser grit (such as 120-grit) and then progress to a finer grit (220-grit), until the wood surface attains the desired smoothness.
* The process of staining can cause the grain of the wood to raise. The raised grain will give you a slight texture. If the raised grain is undesirable, before sanding, soak a sponge in water and wring it out lightly. Wipe the entire surface, making sure that you wet the wood evenly. This ensures that the grain of the wood raises uniformly and will remained raised once dry.
* While you can brush on a stain, many sign makers prefer applying it with a rag or cheesecloth. Varnishes are usually more easily applied with a brush.
* After you apply the stain, make sure that you allow enough drying time before you varnish.
* Safely dispose of your rags in a metal contain with a secure lid. This will prevent fires resulting from spontaneous combustion.