Working With Polyurethane
Foothill Model Works manufactures many of its rolling stock and structure kits our of a substance known as polyurethane. Since this type of material is new or unfamiliar to most model railroaders, we have provided the following tips as an aid to working with polyurethane.
Should any of your parts arrive warped, its easy to straighten them out again. Gently warm the part in hot (about dishwashing temperature) water. After a few minutes, the part will get slightly soft and will conform to any flat surface on which it is placed. At Foothill Model Works, we take extra care in packaging our kits to keep them from warping during shipping. However, the kits are occasionally subjected to extreme heat during shipping, causing the parts to warp. In the unlikely event that any of the parts from our kits cannot be straitened out by this method, we will replace them free of charge.
Washing Parts (Extremely Important!):
Thoroughly wash all of the parts in warm soapy water and with a cleanser (like Comet or Ajax). The mold release used in production will keep the cyanoacrylate adhesive (CA) and/or paint from adhering to the part and must be removed prior to assembly. Cleaning the parts 1st will keep you from spreading the mold release to other parts and to your work area. After you have finished preparing the parts for assembly or painting, its a good idea to wash the parts again. CA and paint also won’t stick to the oil found in finger prints.
At Foothill Model Works, we try not to send out any parts with air bubbles on the front of any part. However, air bubbles are an inevitable part of the casting process and, from time to time, a bubble or two will get past us. The bubble holes are easy to fix with a filler made from baking soda. Fill the hole with a drop of CA adhesive; a toothpick or a scrap piece of wire often works best. Then sprinkle common, household baking soda in the hole. The adhesive will quickly soak into the baking soda and will harden almost immediately. File, sand, or scrape the resulting fill flush with the surface of the model. If the entire hole is not filled with the first application, repeat the process. Larger holes should be filled with several layers of fill to assure that the adhesive is distributed evenly.
Sand the Backs of the Parts:
During the manufacturing process, we press a piece of waxed plate glass on the mold to maintain a uniform thickness and to give the castings a flat back. The wax leaves an impression in the back side of the wall castings. A small amount of sanding with fine sand paper on a sanding block or taped to a piece of glass will remove these impressions. This will improve the appearance of the castings when putting an interior in building kits and help produce a stronger glue joint in all types of polyurethane kits.
Adding Board Scribing and Wood Grain:
Polyurethane can be worked just like styrene. To add scribing lines to simulate the gap between boards simply line up a straight edge or square on the part and scribe with the back edge of a #16 Xacto™ blade. Wood grain can be added by scribing lines in the manner described above freehand for the coarse part of the wood grain. Use a fiberglass scratch brush in the same manner for the fine part of the wood grain. Use 0000 or 000 steel wool to clean up.
Making and Reinforcing Glue Joints:
Gluing polyurethane parts together is easier than you think, even with CA adhesive. To glue two large, flat pieces together, use a few drop of slow-acting CA adhesive or 5-minute epoxy and line up the parts. To glue smaller parts together, clamp the two parts together (with your hands, rubber bands, etc.) and put a few small drops of thin CA adhesive along the joint. The adhesive with wick up into the joint and glue the parts together. If you make a mistake, simple soften the glue joint with M.E.K., separate the parts, and try again. When an extra strong glue joint is required (like the corners of a building without an interior) simply spread a fillet of glue along the joint with a toothpick and sprinkle baking soda along the joint.
All text, images, and drawings ©2008, Foothill Model Works.