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January 26, 2021 3 min read

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.

dyed primer for sanding indicator

  • Make sure not to add too much dye to your indicator layer, as a colored dye in too high of a volume will impede the curing process for your primer. If you reduce your primer, take your colored dye into consideration in calculating volumes.
  • To avoid a reaction with the dye, you can instead use a colored gelcoat to mix with your surface primer, as long as the systems are compatible.
  • The top layer is blocked flat again, followed by a thorough sanding up the ladder* of grit. When the color layer is revealed, it indicates that the primer layer has been removed and the operator should stop sanding in this area. If further sanding is needed however, you may need to re-coat with primer or consider strategic Bondo fills.
    (*sanding through the ladder means to increase your grit from coarsest to finest grit. Once you finish sanding with 180, move up to 220, then to 320, etc. until the finest desired grit is achieved)

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.

  • Hold your sandpaper tight to the flat edge of your block and use a cross-hatching pattern to ensure a level and uniform surface topography. Sanding too much in one direction will leave you with directional scratches that are hard to get out without “stepping down” your grit (stepping down refers to using a rougher sandpaper, one step lower on the ladder).
  • For large flat surfaces, using a long 18”-24” block with a cross technique will maintain a uniform, flat surface.

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 and we may use your experience as a basis for further blog posts. Also, check out ourDIY Blogsfor more information on the processes, projects and various troubleshooting methods found in the Common Fibers shop.