How To DIMENSIONAL
By JIM HINGST
Decorating HDU Priming and painting textured sign blanks for outdoor purposes.
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or anyone who is carving an outdoor painted sign, your best raw material is usually high-density urethane (HDU) sign board. Unlike wood, you do not need to be mindful of the grain direction when carving. For that reason, HDU is much easier to fabricate than wood. What’s more, because this closed-cell synthetic material does not absorb water, it will not rot, decay, warp, or splinter. Working with HDU is relatively easy, however you still need to follow a few simple rules. I will explore three topics to help ensure your success—the selection of HDU sign board, shaping it, and painting and priming it. Selecting HDU Use the highest quality materials that your customer is willing to pay for. Remember: There are no shortcuts to quality and value. High-density urethane is available in a variety of densities. By varying the chemical formulation, the manufacturer can control the degree to which the mix12
Sign Builder Illustrated
ture foams and, consequently, the density of the sheet. Density is the weight of the HDU per cubic foot, and there are various densities. The most common density used in the sign industry is fifteen-pound. This
if you fabricate an hdu sign according to manufacturer instructions, it should last ten to thirty years. heavier material is harder and stronger, which allows you to carve more intricate detail. The eighteen-pound material is also smoother, so it takes
December 2017
fewer coats of primer to achieve a desirable finish. For larger sign projects subjected to significant wind loads, many sign makers select thirty-pound material. Just be aware that the denser HDU costs significantly more. What can I say? You get what you pay for. While signs made from HDU have a number of advantages over redwood and cedar, one disadvantage is that it has neither the density nor structural strength of wood. As a comparison, redwood density is about twenty-seven-pound per cubic foot, which is nearly twice that of fifteen-pound HDU. Whereas wood is comprised of long, sinewy fibers, HDU is made up of very tiny cells (bubbles) that are formed in the chemical foaming process. These little bubbles don’t provide a lot of structural strength. Whichever density you settle upon, thoroughly read the manufacturer’s instructions beforehand. Do not assume that the rules that apply to working with one product, such as an epoxy adhesive or primer from one manufacturer, are the same as those for a similar product. Shaping HDU In cutting and shaping HDU, you can use many of the same types of power tools and hand tools that you would use when working with wood: circular saws, band saws, table saws, jig saws, angle grinders, and routers. When using an angle grinder, be sure to wear your safety equipment—including a tight-fitting respirator with a particulate filter and either a face shield or safety glasses. You should also use earplugs, especially during prolonged use of any power equipment. Once you have the basic forms cut, you can use standard wood carving tools, such as chisels, gouges, knives, rasps, and files. Because the HDU has signshop.com