When the top dries, it is...
When the top dries, it is ready for a coat of fiberglass-reinforced filler.
Before we coat the shaping material with resin, take time to mask off any areas that should stay resin-free, such as the mounting surface of the speaker ring and the sides of the enclosure. A little time spent masking now can save a lot of time sanding and grinding later. Pour and mix a batch of fiberglass resin with the correct amount of MEKP catalyst and apply one light coat over the shaping material. (For thinner shaping materials, the amount of MEKP can be increased by 1 1/2 times for a quicker gel time without running into any problems.) Using a small electric heater fan helps expedite this process. After the first coat gels (or dries), mix up another batch of resin and apply a second thicker coat. It's important to apply a second coat of resin because it allows the shaping material to be fully saturated and will create a stiff surface that makes it easier to apply the fiberglass mat. Although it is possible to apply fiberglass mat before the resin on the shaping material has dried, it is much more difficult and time consuming to work out air bubbles in the mat when working over a soft surface. Once the second coat has dried, use a piece of 50- or 80-grit sandpaper to smooth out any rough spots and bumps that can cause air bubbles under the fiberglass mat.
A batch of reinforced filler...
A batch of reinforced filler with an appropriate amount of cream hardener, ready to be mixed.
Before applying the fiberglass mat, be sure to have all the same supplies and materials ready as we did for our fiberglass shell (in the June 2005 issue). Pour and mix another batch of fiberglass resin (with the proper amount of MEKP) and brush a quick coat over the shell to make it tacky. Apply a layer of shredded fiberglass mat over the entire surface, overlapping the pieces like roofing shingles. Position the pieces of mat so they meet up to the rabbeted edges but do not overlap them. The mat should be placed so that it doesn't hang over the perimeter of the shell and doesn't overlap the taped areas. Even the thinnest fiberglass mat is not flexible enough to contour to the tight edge of the rabbet and will create an air bubble. Applying the mat neatly will reduce the amount of grinding and sanding later. Using a 2" bristle brush, "wet out" the fiberglass mat with resin with a pressing motion so that the mat stays in position. Once the first layer is "wetted out," apply consecutive layers of mat and apply more resin. It's a good idea to apply one full layer of mat at a time rather than partial layers, because it is easier to keep track and creates a more uniform thickness. Work as quickly as possible (without rushing) so that the layers can be applied before the resin kicks and starts to heat up. If the fiberglass shell does kick and heat up, stop applying the mat and wait for it to cool off before starting up again. Once the desired thickness is achieved, allow the shell to harden and cool off. With properly catalyzed high-quality resin, the shell should be hard and dry to the touch in two to three hours.
Smooth out the shape using...
Smooth out the shape using a plastic spreader to fill in any low spots or rough areas.
Although the fiberglass shell is not fully cured until it sits overnight, we will be applying a fiberglass-reinforced filler (e.g., Duraglas/Everglass) to smooth out the shape at this stage so that the resin and filler can both cure overnight. The purpose of applying the reinforced filler is to fill in low spots and get the top of our enclosure closer to the desired shape. Some areas may need quite a bit of filler, while other areas will need very little. Fiberglass-reinforced filler rather than standard body fillers is used for the first stage of shaping because it is stronger and less prone to shrinking and warping, especially when filling areas over 1/4" thick. To prepare the fiberglass top for the filler, use 36- or 50-grit sandpaper to knock down any rough spots and wipe the surface clean of any dust. Thoroughly mix a batch of reinforced filler with the proper amount of cream hardener and apply to the entire surface.