Because we are building solitary parts, rather than manufacturing, the mold need not be complicated. I have made molds from cardboard constructions and blocks of wood glued together and sanded to shape. In most cases you won't get the shiny surface you get from a permanent mold, but you won't have spent the time, money and energy required to make such a mold either. Another strategy is to make the mold from thin plywood and leave the plywood sealed in resin as part of the finished product.
RELEASE IT
With my refrigerator, I got lucky and was able to use the original lid as a starting point, so I didn't need to build a mold from scratch. The heart of the lid was a shallow rectangular pan with one sloping side, which matched the opening in the countertop and allowed the lid to open. I used the pan as both the outer surface and the mold for the fiberglass reinforcement.
Paul EsterleMolds need not be fancy. The author built his from an existing refrigerator lid.
I cut the top and four sides to the exact size required and then beveled the inside edges so the pieces would mate well. I then taped the pieces together to form the pan that acted as the mold for my fiberglass.
If you plan on removing parts from a mold, some sort of mold release will be required. One option is PVS (polyvinyl alcohol), which can be brushed or sprayed on the surface. Heavy applications of wax also will work, but be sure to use a Carnauba-based wax and not a silicone-based product. I have even used melted candle wax as a mold release on small projects.
The next step is usually to brush on a layer of gel coat. Use a laminating type of gel coat that will remain tacky, and aim for a uniform thickness with no bubbles. The mold, together with the tacky gel coat, forms the foundation for the fiberglass reinforcements and resin.
WORKING WITH RESINS
Fiberglass is applied in layers to minimize heat build-up associated with the curing resin.
Polyester resin is available in two basic flavors: laminating and finishing. It is laminating resin that is used to build up the layers. Laminating resin is "air inhibited," meaning it will not cure tack-free when exposed to air. This is a good thing when building up layers of resin and reinforcement, because each succeeding layer bonds both chemically and structurally to the one below it.
However, it also means that polyester resin and its complements are best suited for new construction. Applying new polyester laminations over old severely limits the chemical process, leaving only a structural bond – and even that depends on proper surface preparation. For projects that incorporate existing fiberglass structures, epoxies are often a better option. Polyester resin requires a small amount of catalyst, which triggers the polymerization that will turn the resin from a vicious liquid into a solid material. The amount of catalyst needed depends to a great degree on the ambient temperature of the room. Most resin containers have a chart indicating the proper amount of catalyst that should be used in relation to the ambient temperature.
Using more catalyst than recommended will shorten the "pot life," or working time, before the chemical reaction starts. As it begins to polymerize, the polyester resin will heat and that heat helps cure the resin faster. Large pots of resin with the catalyst added will cure, or "kick," faster so you should begin by working with small batches. Keeping the resin cooler will prolong the pot life, and it is much safer. Large pots of resin can actually "boil over," melt plastic mixing containers or catch on fire. Consider yourself warned.
The catalyst is also very dangerous, especially if you get it in your eyes. Always wear eye protection, as well as proper gloves. Polyester resin also smells really bad and has high levels of volatile organic compounds. Many boat builders have gone away from open laminating and moved to resin infusion or resin injection molding to comply with federal emission standards, those these techniques are more advanced and require expensive equipment.
