I appreciate the constructive input CBG. What density XPS did you use?
Over the years, I have noticed that many who have problems with XPS use low density (1.3 pcf) XPS housing insulation – O-C 150 or Dow Residential Sheathing.
Air/gas content of low density polystyrene foams – XPS or EPS – is high, causing significant expansion and contraction with wide temperature ranges.
Low density EPS needs vents and stiff veneer skins. Because XPS is closed cell, there is no way to vent expanding/contracting gas.
Like EPS, higher density XPS has less gas content and less volume change. One option to reduce post-build expansion might be to allow the shaped blank to heat up in the garage on a hot summer day, allowing the shaped blank to expand before glassing.
Like EPS, XPS is made from polystyrene. As you mentioned, polystyrene has a relatively low surface energy that doesn’t form super strong bonds with epoxy. EPS bonds better because it is porous, not “closed cell” like XPS (very small gas cells).
My test panels are bodyboards. Four years ago, I threw together a quick unglassed XPS bodyboard for the grandkids. Shaped it from 3 sheets of XPS housing insulation bonded with 3M 78 (still holding together). It occurred to me that bodyboard builds were the perfect “test panels” for XPS build tech experiments. Before that, I did standard small test panels.
There are multiple variables affecting delam and bonding.
Polystyrene surface energy.
Appropriate adhesives.
XPS minimum Compressive strength (correct foam density).
Lack of XPS porosity – “small” closed cells. Perforation number, size, shape, orientation, surface area (internal & board surface distribution). Large (relative) circular perforations have low surface area to volume ratios. You want high surface area to volume perforations, many very small holes (Topflite Woodpecker tool, Wartenberg 7-row pinwheel).
Glassing schedule.
Load and impact dispersion (deck skins: EVA, Cork, etc).
Foam temp (internal gas volume) at time of glassing – core and surface layer – affecting post glassing expansion and contraction. XPS does not out gas (closed cell foam cannot). Which is more likely to cause delamination, expansion or contraction?
Hand oils on the surface of non-porous foam, from handling during shaping and glassing. Wear clean or new dishwashing gloves.
Board color – opaque white pigment to reflect solar radiation.
Design and construction must consider these variables.
There are many generalized assumptions when discussing XPS for surfcraft.
(BTW WidgetCo sent me a free cork sample on request. Seemed pretty durable/tough so I bought 2’ x 30’ roll of 2mm for $60 (free shipping.)