xps ding delam help?


Long time no post…

I’ve been making boards for some years now with xps straight from the local lumber yard, and I haven’t had delam issues.

However, I’ve had a couple recent major delams where I’ve done large ding repir/“frankin-remodel.”   I feel like I did everything the way I’ve normally done, and haven’t had problems before.


Thanks for reading.

Taylor O

XPS is notorious for its poor adhesion.    I don’t know what the current glassing protocols are, but in the past when I’ve used it I was finishing the blank as normal then going back with #60 grit to roughen the foam up and cross sand #36 grit on the deckpad areas in order to provide a better bond.  If I was going to use XPS now I think I’d try adding a layer of unwoven veil between blank and the rest of the lamination, just to provide the additional adhesion.  I’ve done that with EPS and PU with great success, so maybe it will also work with XPS.  But then again, maybe not.  I don’t know.   

Last summer 2 of my XPS boards developed large delams. I gave one of those boards to a friend and he rode it a while before it delammed, he still uses it. My board has a large delam on the deck. It was sitting in a shed for a while before I noticed the delam. I think these boards need to have the tiny holes in the lam to allow it to breathe.

I also had an EPS board delam as well, but I think the delam is between peices of foam and not where the glass and foam meet.

I think the climate change here got to my boards. It was a really hot summer in 2019 and 2020. I even have a PU board that has a lot of bubbles from leaving it in the sun.


I don’t think the extra unwoven veil will help. There is something in the XPS that keeps the resin from grabbing the foam. Best thing I’ve found for gripping into XPS is a foaming PU glue like Gorilla Glue. Putting tiny holes in the lam or leaving lots of pin holes helps a lot. I had boards that were glassed after leaving the foam very rough and the glass peeled off easily and it left the foam pretty clean. Here’s one.

Thanks for the replies. 

I’ve been punching holes in the foam, and that seems to help, and 60 grit - I may start putting vent holes everywhere, not just the pressure areas.

I’m using 4x4oz with 4oz 45degree patches on the deck and the fin area on the bottom.  

It was just a bummer, as I’ve had some boards with no issues for years…

If I get my “sharing party” going, I’ll take pics of the blank before it’s too un- boxy…   Maybe some youngster will be inspired.   Then again, maybe I’ll look up the eps company again… Ha!

Whatever else is going on, we can’t allow these boards to get any hotter than it takes to melt the deck wax.   That means no leaving them out in the sun at the beach or in a car during the day.   As long as I kept them inside and in a bag I never had a problem.  I like the weight and density and the ride of XPS; they just seem to be more finicky in terms of care and handling.   

It wasn’t until my kids started taking them out of their bag before they left the house or left them exposed in the bed of their truck that the boards overheated and we ran into delams.  Damn kids.      

I use non-woven veil when I am building fin panels (wood ply+ G10) and IMO there is no comparison in the amount of adhesion you get with woven vs veil.    When I was doing veil under vacuum bagged veneers and trying to peel those off, the (EPS) foam would tear before the glassing let go.  It was impressive.   I don’t know if it’ll work with XPS, so I’d test it out first.   But I think it might be worth a try.   


Try peforating the final shaped blank with a “Wartenberg 7-row Pinwheel.”  Makes lots of small holes (more surface area).  They are fairly inexpensive on eBay.

Run the roller/perforations perpendicular to the long axis of the SB.

Use the highest density XPS you can find.  Glassing the foam while it’s warm may help reduce later expansion in the heat.  Opaque white pigment will help reflect solar energy.

I like to pre-seal with a thin layer of epoxy over the perforated final shape.  Allow to cure.  Then sand with 150 grit before laminating with FG.  Next build will be pre-sealed with resin pigmented with opaque white then FG laminated with opaque white pigmented resin.

Also a cork skin, over and under FG, on the deck should help absorb impacts.   Cork skin should also help insulate the core from heat as well as impacts.

Hi gdaddy,

Would you share more info on the Nonwovens?

Is it glass? The light stuff, I think it’s called C-glass?

Just asking, I am looking for exactly that, a light, and easy drapable Nonwoven to seal eps blanks.




Images of my Wartenberg 7-Row Pinwheels (mentioned above) at this link/post:


Will use alternating rows of Wartenberg and TopFlite Woodpecker perforations for next XPS build.

I done some boards with xps rails, try all the preps from 40grits to many small holes with “fakir bed” tool but had delams where board dings. To stop hemoragy I drill small holes in lam like Javier tech, it’s more or less work…

From what I learn problem come from process use to extruded foam, some specific like spyder foam seems to not have or far less this problem…

For increase bond strength, an alternative to veil is to make a glue paste with some microfibers. 

What density XPS did you use for your rails?

The denser I can find, it was 35 kg/m3. 

Thanks gdaddy,hi taylor0,

sorry for hijacking the thread…

I am curious what you think.

From what I heard, epoxy is know to bond poorly to thermoplastics, so my idea is to use a suitable glue, like 2k pu, to glue a Nonwoven on the xps, letting the outside stay dry to have a coupling layer that is going to bond with epoxy. It would be similar to inmould epoxy lamination…

It’s a double lamination, but could probably be worth it. 

Everything poorly bond to most xps. Pu glue too, and most pu glue seems to loose their strength with time, faster if UV and heat cycle exposed.

On other hand many DiY composits plane builder use epoxy fiberglass hand lam on xps foam shape to build their plane, and they fly with so…  They use blue xps called spyder foam that’s “open cell” in surface even after shape so they use first epoxy micro slurry to reduce weight. I don’t find it here but I find that thinner, 20mm, yellow sheet at home depot are stiffer under thumb and “open cell” surface not glossy like thicker sheets. I bet resin stick better on them, i have to test…

I also see that 3D core, composit sandwich core, is disponible in pet or xps, so xps can work with resin. 

Hi Lemat,

Thanks for your feedback!

Just want to mention that

these sandwichcores are punctured and therefore have a resin bridge. 

I am not sure if we talk about the same PU glue… Pu can be modified in many many ways. 

With the " open cell"… I always thought, that would be the case with all xps?

Being the reason why it doesn’t suck water and resin doesn’t penetrate into the surface?





For all us ugly Americans, 35k/m3 is 2.18lbs/cuft.  This issue with xps vs. eps is a simple one when we look at bonding.  Xps is a blown bubble type of foam consisting of short strands of polystyrene.  The lower the density the larger the bubbles.  When sanding, the surface of some bubbles are worn away leaving depressions that the resin flows into.  The problem is that polystyrene molecules are lighter in molecular weight compared to polyurethane molecules and make really small bubbles even at low (aprox. 1.8lb/cuft) density limiting the contact between resin and foam.  Eps consists of larger balls of polystyrene strands wrapped around a droplet of water, which thru evaporation, is replaced by a blowing agent like liquid pentane that with heat causes the balls to expand and softens the surface of the ball allowing them to adhere to one another.  But, just like pushing pool balls together with a rack on a pool table, there are large gaps between the beads.  (all this can be seen under a low power microscope).  When the beads are shaved down by sanding, there are large gaps exposed between the beads which soak up a lot of resin and the shaved beads themselves have many smaller gaps in their exposed structure creating more surface area for the resin to bond to.  When Marko interred the blank game, they started to fuse their blanks tighter than the available low density block and boards didn’t need to have the voids filled as much but still had plenty of adhesion.  In 2005-06 I saw a variety of Channel Islands experimental boards made out of xps.  There were holes punched in the blank, holes punched in the lam, and other tricks to stop delams.  None worked to the satisfaction of the boss and so xps was abandoned as a material.  If that’s all you have available, then you adapt by trying some of the advise offered above which may or may not work.  Here’s how I view the issue.  If you can get eps or poly, why save a few bucks to make something that will always be inferior?  Why have your name attached to something that fails?  That’s just my 2c…  By the way, poly light foam is around 3 lb/cuft…

Well some guys still work to make it work like Javier at epoxy pro or Fletcher at FCD. For me It’was a short test on rails, like more stay with eps

“Inferior” is relative.  Each type of foam has its own shortcomings.  XPS’ greatest asset is that it doesn’t soak up water when dinged (closed cells; small gas bubbles.)  And contrary to popular myth, it does not “outgas.”

EPS and XPS are made of polystyrene.  Polystyrene has a “surface energy” that is lower than polyurethane – doesn’t bond as well with epoxy.  Physical bonding becomes more critical – with pores serving as resin anchor sites.

For bonding, pore (perforation) size and number are critical.  In general, smaller perforations have a greater surface area to volume ratio (but too small and shallow like sanded XPS cells, resin doesn’t penetrate or bond  well).  A finishing nail hole is not small.  A small thumbtack hole is better.  Also the greater the number of perforations the greater the amount of bonding surface (2 mm apart is much better than 0.5 inch apart).  Pore shape and orientation also affect anchoring.

Don’t use a lightweight glassing schedule.  And/or use impact absorbing skins (e.g. cork) under or between fiberglass.

Handling shaped XPS with bare hands adds skin oils to a non porous surface which affects epoxy bonding.

One of the greatest errors with XPS is using “low density” housing insulation that has low minimum compressive strength, allowing it to crush and delam easily.  Low density foam is also significantly affected by heat expansion and cold contraction.

If you want a board that doesn’t soak up water, you have to do your homework and build it right.  If you want it to last, you have to protect it from extreme temperature changes, while using and storing.

Opaque white pigment helps shield the core from solar heating.

You can’t use inferior build tech.  If you do, XPS is not for you…

Well, this turned into a fun thread.

Lot’s to think about - but I’ll confess, I’ve had boards last a long time that I’ve not taken extra super care with.

A huge thing for me is my big “belly” design, i.e., thick in the middle, much thicker than any blank I’ve seen, and I got burned out on eps dings needing lot’s of drying and digging before I could fix 'em up.  I am thinking of revisiting eps, as, the current delams not necessarily withstanding, I’d also like to move on from glue line action.  And, I realize I can hot wire with my bull nose round corner bead pretty well and do not need a stringer - so that’s a lot less of that step of work.