This is an example of how to build a plug and mold for fiberglass or composites. In this case we were building a longboard skateboard mold from scratch.
We began by cutting out the outline of the shape we wanted from a block of urethane foam (Last-a-foam) and then shaping in the bottom contours of the skateboard that we desired.
This was then mounted onto a sheet of plywood to provide the flange area that we would need on the finished mold to accommodate vacuum bagging materials. This particular board is designed around resin infusion technology and the large flanges make it very easy to fit the resin brake area and inlet and flow media on the part. From past experience we learned that without ample flanges it is more likely that you’ll have problems getting the bag to seal and to get good wetout.
After mounting the foam plug to the plywood we covered the whole thing with a layer of 4oz fiberglass cloth. This gives us a hard shell to work from over the foam and plywood and reduces the probability that we would damage the foam plug. I found the 4oz glass to be a little on the light side and would probably use a heavier cloth or even chop strand mat next time.
Next the part was covered with a polyester body filler putty. We stock and use 3M lightweight body filler. This was primarily to fill the weave of the fiberglass cloth and provide a surface to fair from.
This was sanded, checked, re-puttied, faired, etc until we were happy with the results.
As part of the process we also added a fillet to make a transition from the mold flange to the body of the part. This was done using body filler and radiused tools, such as tongue depressors.
Everything was sanded to 120 grit in preparation of an application of gloss resin. Because gloss resin is viscous there is no need to go finer than 120, it will fill the scratches just fine. Before coating the plug it was wiped down with a tac rag. We use Surgical Blue tac rags because we haven’t had any issues with them causing fish eyes or other surface contamination issues. We have had issues with other brand tac rags. Some people will use clean white cotton rags and solvent wipe. Whatever you do do not use a blow gun to blow it off- that will just redistribute the dust back onto the part (and other areas of the shop, you, etc).
We used gloss resin to coat the plug because it cures to a hard film that can be wet sanded and buffed to a high gloss. We added black pigment to the resin so that we would be able to see the quality of the surface in the light and to hide the visual busy-ness of the underlying materials.
After mixing the pigment into the resin, usually 3% pigment by volume will give you opacity (about 4 oz per gallon). The resin was strained to remove any impurities then initiator is added to the resin.
The brushing technique that was used here is the same as the one used to hotcoat a surfboard. The resin is poured out and spread over the plug, then using quite a bit of pressure is brushed at opposing 45 degree angles to even out the layer of resin on the plug, finally it is lightly brushed along 1 axis to smooth it out. From there gravity takes over and the resin will level out. As it cures wax will rise to the surface and the surface sheen will dull.