Composite fiberglass/foam panel build question

[stoneburner]

All very good points. But using higher density XPS doesn't solve the issue of outgassing. There was a camper build on this site where the builder experienced delamination due to heat (first time the camper saw the sun). It was sanded (only) 60 psi XPS. Outgassing of XPS at elevated temperatures can cause this. I've heard of one surfboard builder who perforates the FG skin on his boards with tiny needles. But I suspect that delamination of this sort can be eliminated by ensuring a good bond.

Higher density XPS does not crush or shear/tear as easily as low density XPS. Because it has higher density there is less potential for expansion too.
XPS should not outgas. It only aborbs 0.1%-0.3% moisture by volume because all cells are completely closed. Gas does not circulate freely withing XPS foam like it does in EPS foam. Gas can only be released by the thin layer of XPS surface cells if they rupture -- crushing or tearing. When using EPS, vent are often used to adjust internal pressure when the air between beads expands with heat.
However, the XPS panel will expand and contract with extremes of heating and cooling -- influenced by density. This can be enough to cause the FG skin to separate from the foam.
One pro board builder made an XPS board sanding to only 150 or finer, used a thin coat of epoxy to seal the foam and then laminated. Said the board lasted for years without delams.
Those who use XPS for board builds protect those boards from heating (XPS softens with high heat -- in or on top of a car). Upper limit of service temp is 165 F.
Also it would be wise to use light color pigment for XPS skins (e.g. white) to reflect rather than absorb solar energy.
Perforation is used to improve the bond and reduce surface skin movements. Hydroflex surfboards patented their perforation tech.
My next build for body boards will use 25 psi OC 250. I will perforate using a WPT with 2x standard tyne number and sand to 150 grit. I will use specific perforation (lengthwise) slit orientations relative to flex axis. I may use opaque white pigment in the bottom epoxy. The deck will be covered with 1/4" EVA which should proved some insulation from heat. But the modified WPT and slit orientation will be my main method to improve bonding.

 

[rruff]

using a WPT with 2x standard tyne number and sanding to 150 grit and specific perforation slit orientation relative to flex axis.

Can you explain that more?

 

[stoneburner]

The spacers/washers and tynes of the TopFlite WoodPecker tool can be removed and/or added. A couple of Christmases ago the online retailer offered free shipping on their WPTs. I bought 2 WPTs so I could use the tynes from one to double the number of tynes in the other (twice as many perforations per square inch). The tynes are about as thick as a single-edged razor blade.

Because the tynes are flat they make incisions that are longer than they are wide -- linear incisions in the foam. So you can run the slit's long axis laterally, longitudinally or at angle to the length or width of your XPS panel (in my case blank). For surfcraft, the blank flexes relative to the long axis and also flexes relative to the width (twisting). So with the WPT, I can run slits parallel, perpendicular or at angle to the long axis of the surfboard.

I will final sand the XPS surface with 150 grit before perforating this time. Remember I am still experimenting -- but feeling good about this approach.

EDIT: It is important to sand and vacuum the dust off the XPS before perforating. You don't want to fill the peforations with dust.

 

[rruff]

Have you tried the dog brush?

 

[stoneburner]

No. Looks like it would create too much tearing/roughness of the foam which would make the final laminated surface too rough for surfboard aesthetics.
The WPT rolls almost effortlessly over the XPS surface and perforates the foam readily with many fine peforations. In every case, the FG cloth would snap while peeling it off the XPS. The final WPT peforated surface is smooth.

EDIT: A new WPT comes with tynes and spacers coated with some type of oil. Epoxy will not bond to foam with oil residue in the perforations and on the surface. Be sure your perforating tool has been thoroughly cleaned to remove oil & petroleum based compounds from the tynes (& WPT spacers too).