I want to do some non-structural board mods and have a bit of XPS laying around (pink insulating foam in US). I feel like I recall someone here saying that epoxy doesn't bond as well to it but it's at hand and pretty easy to work with. I've used it before but that project hasn't seen extended use so I don't know how well it holds up long term.

Anything I should consider before I start gluing chunks of it onto perfectly good boards? Doing some bottom mods and extending the deck by the back straps.

Alternatively, I have some 2 lb pour foam at hand as well.

Roughen the surface with 40 grit and it bonds ok.
I glue blocks into the blank for more waterproof footstrap mounting areas.
I recently glued a test piece to EPS trying Gorilla glue and the bond is very good.

I have used the XPS Dow blue colored version. I think it is the same as the pink which is made by Owens Corning. Never had any problem with cloth adhesion. XPS is closed cell and probably absorbs less resin than EPS. The outer surfaces are smooth from the extrusion process and possibly may have been the reason for adhesion issues.

Once I used a 4 inch thick piece to increase the width of a board by 3 inches. I cut the board in half lengthwise and removed the middle inch to get rid of the boxes. I then epoxied the 2 halves to the smooth surfaces of the foam and faired the top and bottom into the existing board. I have never laminated cloth directly to the smooth surface. Like Te Hau said above I would roughen up the smooth surfaces rather than laminating directly to it.

I've built boards out of the blue stuff with no adhesion issues decades down the track, but as said that was on shaped foam not the an off the hot wire shiny surface.

Its not an adhesion issue, its the fact the XPS sometimes lets out the gasses used to foam it, and it creates delam. Particularly in hot climates. But the gases can't move from one part of the board to another like EPS, as there is no interstices. Its an issue if the XPS cracks (like just from use underfoot) and some of that gas comes out then wants to bubble under your laminate. So fine if the surface has been abraded well as Te Hau said., as you're releasing the gas as much as possible in the top layer.
Some surfboard guys using it would roll it with a roller with many sharp pins on it so its been pricked a million times over the whole board, dunno if that helped. To my mind it would. I've never done that but I don't really use XPS for these reasons, and its hard to hard to shape well.
No delam on a surfboard I made with it - but its white, hasn't been in sun much and very very little use as it was one of my first and its bloody awful

Thanks, everyone. Mark, the amount I'll be using shouldn't be enough to produce significant amount of gas so I think I'll be alright. I've shaped it before and I'm comfortable with it although, as you suggest, it's not as easy as EPS.

Still mulling it over. Getting decent EPS with jumping through a bunch of hoops here is difficult.

I can never remember if the E is for expanded and X for extruded or vica versa...

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Jethrow said..
I can never remember if the E is for expanded and X for extruded or vica versa...

EPX is expanded and XPS is extruded. White styrofoam is EPX and open cell. The blue and pink foams are extruded.

Paducah,
This guy has a YT channel on shaping, and he built a surfboard using XPS panels bonded with polyurethane wood glue.
He has 19 episodes if you've got the time. The gluing of XPS panels to build the blank is in this one:

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JCBoston said..
Paducah,
This guy has a YT channel on shaping, and he built a surfboard using XPS panels bonded with polyurethane wood glue.
He has 19 episodes if you've got the time.

No, but that's never kept me out of a yt rabbit hole.

Did read a bit on the topic of off-gassing that Mark_australia referred to. It seems that it's enough that XPS's R-rating (insulation value) actually derates over time as the gas is replaced by air. I don't think I'm going to be using a big enough chunk of it to be significant but definitely something to be aware of if you are building a whole board in a hot climate. I imagine it might be useful to consider a Gore-tex valve for that situation despite their other drawbacks.

Edit: Great series of videos. The money shot at 15:07 and right after: (paraphrased) You want a rough surface for the epoxy to stick because XPS has a tendency to delaminate.

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