Hey all. I’m 17 and currently building my very first surfboard. Yesterday I was gluing and clamping my pre-rockered stringer into the middle of my 2 blocks of foam. I needed to bend the nose of the foam slightly so the rocker would fit in. Unfortunately one block of foam snapped about 1.5’ down. I was wondering if I might be able to cut off both sides of where it broke and piece it back together by means of a 6" overlapping joint (my foam is two 2" thick foam pieces glued together (4"s total)). Does anyone know if it would work? It is only for the core and will now be using it at the back of my board. Shaping it should be fine, I just want to know if anything bad might happen once the board is all done and laminated. Will it snap at that point? Thanks for the help.
Pictures would help…but without seeing it I’m assuming its just got a crack perpendicular to the stringer? if thats the case i say just go ahead and shape it… fill the crack with spackle before glassing. Any attempt to repair it with splices will result in difficult to shape glue lines (which will result bumps and dips in your shape). don’t be too concerned, once its glassed it will hold up, the foam is more of a filler between the glass, the glass and stringer form the structure itself.
Here are some pictures. 
Yes, it snapped perpendicular to the edge where the stringer will be glued (right edge). This goes on the left side of the stringer, I have an identical block for the right, so i simply need to fix the break and glue the stringer in…again. Will the 6" overlap work? Anyways to fix the break which wont affect my surboard after is all done and laminated?
If you do glue it back together then the glue join should be slightly weaker than the foam itself. If it is stronger than the foam and you bend it too much it will snap again in another location (probably just past the glue join) and then you’ll have another repair to do 
rif.
polyurethane glue and clamps. No worries.
Thanks for the reply’s and help. So I should just spread some glue on the jagged edges and clamp them together, and it wont snap there when I use the board right?
Hi Freshwater -
Gorilla glue should do the trick. Try to keep the glue away from the surface of your finished shape to avoid the glue lines mentioned. I.E. just a couple blobs of glue in the core of the break should hold it together pretty well. As you have already found out, light density EPS is weak. It will almost always break somewhere other than the glue joint even if the joint is not filled completely with glue.
After spackling, especially if you use epoxy/microballoon slurry, the joint will be invisible and strong. Force the slurry into the joint after the shape is finished and it will add some strength. The glue joint will be the strongest part of the core.
I’ve seen examples of nice boards from EPS blanks that have been spliced together from the beginning to create a long enough blank. Once the pieces were glued, stringers inserted and the finished shape glassed, it was all good.
I wouldn’t even scarf the joint, just glue the blank back together at the break.
I’m thinking Benny1 can offer confirmation if he sees this.
Hey John. I’m going to head home and do that. Thanks for the info. As for the spackling, what would that consist of? Thanks again
Hi Freshwater -
The slurry is made from epoxy and microballoons. It needs to be scuffed up a bit before laminating if you let it cure completely. Microballoons is a light weight powdery filler you mix in to thicken the epoxy resin. Some guys use an acrylic based spackle but for the added strength you might want the epoxy slurry. There is a lot of info in the archives on sealing EPS and to be honest, there are some different opinions on the best method. Try “microballoons” in the search box.
You might also check the S-Foam and Segway websites for additional sealing and glassing tips… note that S-Foam recommends epoxy slurry while Segway recommends Dap Fast and Final.
Saw the break. Don’t try to cut it straight. Glue it as is. The rough pattern will help you line it up.
I’ve glued up with both PU foaming glue (gorilla glue and their competitors) and epoxy. Both will work fine and you’ll be the only one that knew it happened. If you are still going to use more hotwire, go for the PU glue. I use epoxy when I already have leftover resin that is already mixed up. Just remember not to go crazy with the PU glue. It expands almost too well.
Yes, keep it rough. More gluing surfaces and easier to align. So true about the foaming PU. Just a little on one side of the break and then mist the other side with a kitchen sprayer. Put them together with good masking tape around the whole line & then some pieces stretched nice & tight perpendicular to the crack, long pieces like 16" at least. The green 3m 233+ tape is the best if you have some. Make sure you tape bothsides evenly, if one side’s tighter than the other, the glue will open the crack to the looser side as it expands & cures and you will get an ugly joint.
I like the Elmer’s Ultimate, its cheaper than Gorilla and dries a lighter yellow, the GG can dry orange/brown. Hotwires will go through ok, a little slower than the surrounding EPS. It will shape & sand fine.
Works for putting Surftechs back together too 
Thanks for all the help. I glued it up just yesterday with my Elmers Pro Bond Wood glue and it worked so far. We’ll have to see if it holds up in the shaping. Thanks again.