Polyurethane glue & fiberglass

I just did some tests of different sandwich layups and thought i’d post some results.

I did 4 tests panels which consisted of eps foam,adhesive(glass),corecell

  1. nonsealed eps/epoxy & 4oz glass squeegeed until weave showing/corecell

  2. nonsealed eps/epoxy & 4oz glass with enough epoxy to cover weave/corecell

  3. nonsealed eps/polyurethane glue/corecell

  4. nonsealed eps/polyurethane glue & 4oz glass/corecell

Panel 3 had good bond but no dent strength at all.

Panel 1 had very little bond between the corecell and glass but good bond to eps

Panel 2 had good bond to both corecell and glass

Panel 4 had the best properties of bond and dent strength

This leaves me to believe the expansion properties of the polyurethane glue in combination with glass my be a good alternative to epoxy for bonding the interior of a sandwich board. Because the glue is much thicker than epoxy and starts to swell quickly I don’t think the foam will suck it up like epoxy therefore allowing you to use less glue.

Has anyone tried this yet?

I also did a test with bending corecell around rails. I could get it to bend easily if i glassed the outside first before bending. I could get it to bend aroud a 1" diam. dowel with no problems.


Wait a minute…you put dry glass cloth & foaming pu glue together?

You’re not saying you laminated glass to the corecell with epoxy, and then glued the resulting panel to the EPS with pu glue?

If the former, I have to honestly say I’d never considered that at all. I’ve done plenty of your other 3 - tests and whole boards, with balsa, bamboo, and d-cell, not corecell…but never thought of putting cloth with anything other than fiberglassing resin of some kind. Did the cloth stiffen up? You know, like if any little strands are sticking out the edges of your panels, can you tell if they got individually coated the way they do with cured resin?

How much stiffer was the pu glued panel with the cloth than the one without the cloth? I mean, is there any way to quantify it - like a 12" panel with the ends supported and a brick in the middle bends 1" without the cloth and 5/8" with the cloth?

I’d found it was all about the sandwich material - d-cell & bamboo work best with PU glue on EPS and cloth only over the outside (not true sandwich skins) but balsa likes glass & epoxy under.

Hi Benny,

Yes, I replaced the epoxy with pu glue and it seem to work quite well. The glass stiffened up, not brittle like epoxy, but quite hard. The glass wet out clear (yellow actually) like epoxy.

The tests I was doing was more of an impact test with a hamer. The pu glue without glass was horrible the corecell fractured completley and pushed right into the eps. With glass and pu glue it could take a pounding with only minor dents but no fractures. As far as overall stiffness of a board its hard to tell unless i did full panels sandwiched on both sides.


Did you spread the pu glue with a glassing squeegee? (That’s what I use for EPS joins, stringers, bamboo, etc.)

Did you lay on the glass first & then pour on the pu & spread it, or did you spread around the pu & then lay down the glass? Seems like the pu is pretty thick for the first way, it would wrinkle & fold the cloth as you tried to spread it around…

Did you do these under vacuum or under a weight or just sitting there?

Great information, thanks!

I spread it on the same way as I would have with epoxy. It soaked through the glass nicely (may a little convincing but not much).


Hi Bill,

I was wondering what brand of PU glue you used.


I believe it’s elmer’s glue (Got it at homedepot in canada) not sure if its available in US. I think its the same stuff a gorilla glue but half the price. I came in a large bottle (beer bottle size) for about 17$ CAN. which is probably about 85 cents US.


Good news, good news…That’s the same Elmer’s Ultimate that I already use. Not only half as expensive as Gorilla, but half as brown as well :slight_smile:

One last question…glass cloth weight? Big 8 oz volan to let the glue through the weave? Tiny, thin 2 oz satin? Standard 4 oz E?

Maybe something in your observations led you to conclude that, “I used this cloth, but I think that cloth would be even better because ___________.”

Getting pretty stoked to experiment a little myself. I’m already attaching my balsa rails with the stuff - adding glass couldn’t hurt with durability & maybe a little ‘twang’…

Thanks again

I only tried with standard 4oz E glass and dont think anything heavier would serve much purpose. Lighter may be ok but don’t think that it would be significant weight savings. I think at this point that epoxy/polyester resin is just the binder for the glass which is were the strenght comes from. Substituting the binder has a little/no effect on the glass strength plus you get the benifits of the expansion of the glue which i believe is very benificial when used with pourous foam.


Interesting stuff there bill.

only problem is, youre gonna have a REAL tough time controlling that layup on the entire surface of a large panel like a surfboard.

If you can slow down the pu reaction then yeah doable…maybe…spreading pu glue is not easy either.

what you may want to try is pu adhesive that doesnt foam…some choices there but very few with low easily workable viscosity. The ones that are low visc are full of toxic shit.

Glad your experimenting tho…its always nice to know there are others trying new and different things.


Mee - I don’t think he’s trying to put it on the outside of anything. Just as an adhesive layer + strength for under his corecell skin. Probably regular glassing (epoxy) on the outside.

I just recently tried glossing with a paint-on polyurethane 2-pac. It set up softer - almost rubbery - than I’d like, but I think it has some promise. Need to go much thinner with it, and probably heat it up first too.

Edit: wanted to stay away from 2-pacs, especially in aerosol. This one is remarkably benign, so I guess I should give it a plug: http://www.systemthree.com/msds/MSDS_WR-LPU.pdf Probably why it didn’t finish as hard as others :slight_smile:

But now I’m going to have to go home & try putting some of it into glass cloth, as well as trying the Elmer’s Ultimate like Bill did…

Oh well. It beats mowing the lawn :stuck_out_tongue:

one idea is to squeegee a very light layer on the EPS and Corcell or external skin then lightly soak you glass in water squuezing as much out as possible and unrolling it onto the EPS smoothing it out them laying your external skin over the top and bagging the whole thing.

with this technique you could easily skin the top and bottom at once and bag it all together using any over flow to seal the rail seem between the two external skin panels.

I kind of like the idea cause you can really get a thin layer of the glue on the dry EPS and Corecel panel and have the foaming action push both ways through the glass getting good multi directional penetration of the fibers.

Very intersting visual if you think of the foam action of the glue pushing micro polyuretane stalagtites(sp) thru the weave and into the panel on the other side of the glass at ninety degrees to direction of the fibers almost creating a mirco PU honeycomb effect. Seems like it would be very stable and resistant to hammering but I don’t know how it would affect the overall flex response.

Good way to save the epoxy for the outside layer.

I always like the idea of a foaming binding agent under the skin to act as a shock absorber between the skin and the core and making the skin as tough as possible. Seems like Bert was saying that the more you can unload the stress from the opposite side of the applied torque and the quicker you do this the less chance of you have of snapping your board like John did.

Interesting that there’s no need for a chemical bond between the glass and the resin but that it’s more a mechanical relationship that provides the strength. Any CE’s here to discuss the need to combine resin ith glass versus anything else?

I’m almost envisioning sheets of electrostatically reactive silica fibers that relax and stiffen via induced current or charge. I’ve seen this demoed before… metallic springs that soften and stiffen with the passing of a charge.

I’m almost envisioning sheets of electrostatically reactive silica fibers that relax and stiffen via induced current or charge. I’ve seen this demoed before… metallic springs that soften and stiffen with the passing of a charge.

Hi Bernie -

My girlfriend has something like that in her snow skis…