When you are creating a mold from a machined foam block, an essential step before spraying gelcoat is prepping your primer surface to the desired finish. Any texture on the primer layer will transfer to the gelcoat, so if you desire a flawless mold finish, the plug should be sanded to a high grit and polished to a mirror finish. One of the drawbacks of working with primer on foam is the tendency to sand through your primer on proud features such as raised corners or countersunk bolt locations. If you sand through to foam, you will need to either re-coating with primer, fill with Bondo or filler, or gamble on wax releasing your foam in the hopes that the mold will come cleanly off the plug (let me help you here, it probably won’t). So, what can you do to prevent using your time this way?
At Common Fibers, we employ a layer of colored primer to create a visual indicator for how far you have sanded. This layer is left to dry, blocked flat, then sprayed again with un-dyed primer.
One of the most important things to remember while sanding plugs, and flat parts in general, is to use a firm flat block whenever possible. The block will prevent you from sanding unevenly and creating unwanted ripples by applying pressure evenly over your working area.
An indicator layer and a hard block are two changes to your workflow that will prevent you from over working and re-working your plug, so that you can have a better mold surface and move to gelcoat with relative ease and far fewer headaches.
If you have faced questionable situations in your own productions and have experience to share, email me at firstname.lastname@example.org and we may use your experience as a basis for further blog posts. Also, check out our DIY Blogs for more information on the processes, projects and various troubleshooting methods found in the Common Fibers shop.