I scored this XPS foam block from my local hardware for $20, it’s tongue and groove, some kind of construction insulation maybe? It’s 2.5m x .6m x 100mm thick so it fits the bill, is this good for a blank? I saw some specs on foam here somewhere but can’t find anything on this piece, I don’t want to waste money if it will be no good, cheers
Ok, I looked back, I understand what I’m working with now,
The cheapest residential housing XPS insulation has a minimum compressive strength of 14 psi (crushes easily, lots of pressure dings that lead to delams and/or board snapping).
I would not use XPS foam with less than 25 psi min. comp. strength (DuPont blue and O-C pink are available with this compresssive strength).
Surface preparation before glassing is critical.
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I can’t tell what strength this piece is, I’m going to try and pressure test it,
What board do you want to shape?
You can make a board with but need to take care to low adhesion with resin and degazing of crushed cell against skins when dent. To use it with durable success you should use stiff skins (like sandwich build) at least on deck under feet and prep foam right to have most adhesion possible. Compagny in wingfoil and kite world have success using HD xps even i see major delam on some of those boards always start at an impact dent/ding. Poke holes xtr is an other way to make this foam work, let crushed cell open degazing outside.
Thanks Lemat, I understand, il make a mid length board, I can work with a sandwich where you stand and I understand surface prep with coarse sandpaper and I am working a lot with epoxy, I might do a fabric inlay on the deck as well, il think on it, I like the challenge, and the board will be a fun learning challenge
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So what’s your plan for “rocker”? Or is this the classic flat bottom to concave?
I was thinking, of making a template for either side and wire cutting, I have made some big RC glider wings with wire cutting,
I think if I rip into it with a plainer, the whole lot will end up in the bin 
That is what I do, two matching hardboard templates, screwed into the foam and bridged with a hot wire in a spring-loaded bow…
I would like to try the method using metal bobbins and MIG welding tips that rolls on the templates to avoid the ‘twangs’ one gets when the wire catches on a template and releases, scarring the foam.
Some of the RC guys rig up weights and such so the cuts are not affected by arm movements.
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I scored 10 slabs of XPS many years ago. Each slab was 8’ x 24" x 3" and I paid $10 USD for each one. Some were weather damaged and some were like new. I made several boards from the slabs using them as they were and then I started cutting them into rocker slices to get more rocker. On one of the boards I did, I put a weight on the nose when I laminated the bottom to get more rocker. I just used an unmodified electric wood planer (I still use that planer a lot) and cut out the shape.
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Thanks shark, did you put in a stringer? Your boards look great, I’ve got a pretty good eye and time so I might have a go at using just a plane,
I hope you post a progressive thread with pictures!
You did a really nice job with that project!
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No stringer. I do a lot of stringerless boards.
Alright, so I’ve started on the XPS board, I’ve shaped and got some glass on and I’m happy with everything but I’m trying to decide if a 9" flex fin from blended curves will cut the mustard? I’m a down the line surfer at best with wheelbarrow style turns, is a flex fin the best or should I go for a fuller fin? It’s 7’5" x 22 1/2" x 2 3/4" . I could surf once and hopefully again but after a 15yr hiatus I’m almost back to learning, funny enough when it was big I managed a late drop on my 9’ mal, it’s still in there somewhere
When I said it was big, it was only a 6’ beachy in Tasmania, not big big.
Looks good!! Are you going to glass the fin on or install a fin box?
I’m going to glass on,
I’m planning a plywood with carbon fiber vacuumed on with the missus kitchen cryovac, it has worked before