Needs something under it / a vac kit to get the most out of EPS? Also do you have a favourite EPS manufacturer? Some manufacturers are good for spackeless? Stiffness issues? Any and all thoughts related to eps foam / blanks, hand lay ups / schedules, stringerless / stringered… welcome. Always lift the container of spackle off the shelf before you buy. If it feels heavy its going to be like rubber to apply it. I like to thin spackle with Klear floor polish. Easy to get a hold of (supermarket) and has no solvent in it :). Tap water can make spackle go slightly brown.
Light eps and sandwich skins can make the best lightness/strengh ratio but higher cost and far difficult to make. Even if you make it overall flexible, hard skins feel stiff.
Epoxy spackle is stronger than acrylic spackle but take more time to do, cost much.
Denser eps is stiffer and less spongy, need less glass for breaking strengh but dent a lot and dents don’t spring back. With less fiber, less resin, cheaper board to make. Became a standard build shit. Should stay by far with pu/pe at end more durable with better feel. I am en eps/epoxy guy, I make my board to feel good, even not pu/pe feel, with an higher strengh/weight ratio.
Stringer is one way to add longitudinal stiffness if you want/need it.
The lower the EPS foam density (pcf, pound/ft^3), the greater the foam’s air content.
The greater the air content the greater the internal pressure of a glassed board on the hot sand or on the roof of a hot car, because the heated air expands. Expanded air in the core = delam potential.
And the greater the air content, the lower the internal pressure (vacuum) in cool/cold water – small holes/dings suck in water.
Vents may be needed if low-density foam is used.
Lower EPS densities need sealing (spackle, slurries, etc.) to limit too much resin from being soaked up and air bubbles when laminating.