Preparing foam for glassing with vinyl ester resin or epoxy involves similar steps, but with a critical difference. The result is a lighter finished part as is typical in aircraft construction. Although foam panels rarely encounter peel loads, glass applied over a properly prepared surface offers significantly higher resistance to peeling.
Most foam panels arrive with fine dust residue from the factory thickness cut; vacuum this off before beginning.
Mix a slurry with resin and lightweight filler (microballoons or “micro,” Q-cells) with a consistency of slightly runny peanut butter for vinyl ester resin and a bit thicker for epoxy.
When using vinyl ester or polyester, apply a “hot coat” to the foam to optimize the bond. This consists of resin catalyzed at 2%. Because a 2% batch cures faster than a standard 1% mix, it is known as a “hot mix,” giving the process its name. In early foam formulations, this rapid cure prevented esters from reacting with the foam. In modern construction, this coating fills surface voids. Once the hot coat cures, lightly scuff it to remove any nibs, creating a flat surface for the glass.
The process for epoxy is similar, with one major exception: The filler must not cure before glassing. Squeegee the slurry into the foam surface in two or more directions to ensure full penetration, similar to filling a drywall screw hole. Since the epoxy filler remains wet when you move to apply the first layer of glass, use a slightly thicker mixture to prevent the laminating resin from softening or “diluting” the filler. Regardless of the resin type, wipe the filler clean to leave a smooth surface with no excess piles or ripples. This barrier also prevents the laminating resin from draining into the foam voids, resulting in a lighter finished product.
When you’ve prepared the surface of the foam core, the next step is applying glass.
