Can it work or would it delam without glass?
I can’t say I have access to anywhere anymore to be able to try it 
You could just gorilla glue it. What are you trying to accomplish?
3M 78 spray adhesive should work without a vac bag. Vac bagging might give a little more complete final contact surface.
A plywood pre-seal with epoxy might improve the plywood contact surface.
Get a small piece of XPS and marine ply and do a test.
What are you hoping to make?
If you want to glue ply to XPS use a foaming PU glue like Gorrilla glue and make sure you cover the whole foam to avoid air pockets. The gorilla glue will hold, but the wood will need a sealer. Air pockets will be where a delam will start.
Thanks for the timely advice sharkcountry. I’m about to start something along similar lines, and this is a good reminder for me to be thorough.
Cheers
Rohan
I do this a lot, I used to use marine ply but the only difference with normal ply is that the glue is waterproof in the marine ply. So I use standard 3 or 5 ply. As long as you glass or resin it, it’ll be fine.
I made my first foil shortboard with ply and then a lot of ply foils with just resin.
And more recently ply on EPS and then carbon and glass.
The ply doesn’t flex or twist once it’s glassed.
Interesting surffoil, i just read a studdy about use of plywood with composit skin for transport industry. Guy start from old time when ply was used material for plane, train, boat, car etc to now where it come back more and more, cover with composites, in all those application under green pressure.
In this studdies he compare tech to find the best one to build panels with predictable performances, he find epoxy and pressure…
And he show Amazing impact,fatigue flexural performances for low price but a bit more weight than pvc sandwichs. But there is room to improve…
Lemat, I thought Id struck gold with vaccing ply, I did 3 layers of thin veneer with glass in between and it’s like steel. Used as a structural component it’s unbeatable, rigid but flexible under great pressure. Add an EPS layer and you have a solid backing that can be foiled. I’ve used this technique for fins, foils and now as a bridge between surfboard sections. Sadly the weakest link in construction now is the traditional glass over foam.
Never like working with PVC foam, pricy, ugly stiff but fragile, so i play with wood, balsa samba local frêne and i find like you that multi layers of thin veneer with light glass between infuse with epoxy give very good results, strengh, stiffness, impact and puncture résistance.
Depending on what the OP is making, I’d opt for using PU foam over XPS. PU is still closed cell, it’s more stable at hotter temps and when going from cool-to-warm and warm-to-cool. Shear strength is better and so is adhesion. Unless the piece is large in size the weight difference won’t be noticable.
I liked PU but as a residential board builder the heady aroma of poly was a problem. Here’s my outdoor shaping / glassing / sanding area and you can see the neighbours home is just behind the hedge. He surfs but his wife is a shrew who complained bitterly every time I open a tin of poly.
Now I haven’t used PU or poly for 15 maybe 20 years. Another benefit is that I can scrounge EPS blocks and sheets from appliance deliveries and never pay for a blank. Recycling EPS isn’t always pretty but I like Free.
My best T shirts say…
If it’s free, it’s for me, and I’ll have 3.
You’ll get paid when I get paid.
Stranger things have happened at sea.
I surf like Curren.
This one always gets a second look from grizzly old boomers wearing flip flops in CostCo.
Okay, first, it can work just fine without a intermediate epoxy/glass layer, as everybody has already said. Indeed, the glass might well complicate/bugger up things where a simple adhesive would work fine and have more than enough strength to hold foam to the point where the foam failed rather than the adhesive.
But my question is, what’s the application? What are you making?
Reason I’m asking is that they do make ply/foam/ply sheets already, off the shelf. Very strong/stiff/light, if you were doing flat panels it would maybe be more cost and time effective to buy the stuff rather than fabricate your own. It gets used in the boatbuilding biz for things like casting platforms and similar, where you need something light, flat but solid.
hope that’s of use
doc…
For sure most adhésif can be effective to glue ply on light foam but some stiff fiber can help to keep adhésif at right place if it’s fluide epoxy one’s and play inner skin sandwich effect. I like the glue fiber on wood first with spray glue, help working with thin veneer.
Multiple uses. One of which is to tow it behind a boat and test out various shapes for personal feel to help me learn, maybe not even standing up. Minimal strength needed.
Sounds like I could just take a sheet of ordinary ply, coat it in epoxy without any glass and then epoxy the XPS to the ply.
Not sure how to recycle or dispose of it after though? Drill a hole & put some mealworms inside for the XPS. But then what to do with the epoxy and ply? It’s toxic to burn, right? Will a carbon filter handle that?