There is a old thread about structural repairs, which makes some good points, but I thought I would start another one, as I have a question.
Basically I understand that trying to recreate the laminate, with a scarf joint, like the board lady suggests, is a pretty good effort to recreate the original structure, but it doesn't actually recreate the structure, it's a compromise.
i.e. the bottom layer of glass, joins the existing bottom layer, for perhaps a few mm, before creating another mini layer of glass/pvc/glass. Then the same is repeated for the top layer. I am guessing a lot of the strength comes from the very small overlaps of glass/pvc/glass, rather a glass to glass connection, which is a few mm at the most.
So my question is why not do a step, reomoving as much of pvc, so that more of the existing glass is exposed, providing a greater bond area for glass to glass? even if there is a thin layer of pvc still attached to the glass.
Can this be achieved with a dremel, mini router etc, set to the depth of the pvc?
IMHO --- and if I understand the questions correctly
Whilst its a compromise it seems to be an acceptable one as the scarf joint bevelled repair does not seem to fail any more than the existing (good) material in other areas fails.
You could use the dremel like a router to remove only the divinycell and leave the thin layer of glass under it to create a lap joint on that glass. However I reckon it would be so so difficult as to be a waste of time I reckon. The precision required would be plus or minus 0.5mm ish which is hard on a curved surface.
Reason I say that is I use dremel setup like a router to cut out the divinycell layer around a delam etc, and then lift off the layer. The depth of cut varies quite a bit just due to board shape, some bits 3mm, some bits 5mm. That does not matter as it is then bevelled and some qcell mix fills the low bits.
So short version - a very hard (but no doubt possible) extra step to get no more strength??? Or unnecessary extra strength that is only a minor gain?
Alternative is Barn's old way of tucking glass under with a spatula to make a small lap joint, but I won't start that argument again 
Select to expand quoteMark _australia said..
IMHO --- and if I understand the questions correctly
Yes, that's what I meant. The two advantages was improved strength, the existing structure failed fairly easily (Slalom Board), and reducing the size of the repair area. I understood to do a proper scarf you need a 1:30 taper ?, so the area needs to be fairly large, the later is the primary motivation.
How difficult was it remove the pvc layer with the dremel? what attachment did you use?
Thanks
Dremel (and my old dodgy Ryobi copy) has a collar that screws on the front so it can be used as router. Looking for a pic...
Then the bit is like a drill bit with sharp edges on the helix, its a cheap version of a mill (ie: for a milling machine) and it cuts pretty good.
That came free with my Dremel BUT if you have to buy one then don't, for an extra $10 you get a proper plunge router type thing for it.
It is easy to use that to cut out a buggered area like I do, as the sandwich is delaminated and it lifts away easily. But as I said to only remove the pvc and leave the glass on top of the styro could be really hard.
Are u sure about 1:30 taper? Never seen a chippie do 30" on a 1" thick piece of wood..... maybe 1:3? Or maybe 1:30 is a technical ideal and not done often... ?
Anyway I bevel out a deck sandwich over about 20mm max and after a few big repairs in between the straps where it is mega stressed, those boards have been jumped hard for years afterwards and still good. I think u are maybe overthinking it.
Thanks mark! I have a dremel, with many of the attachments, including the one you posted.
The 1:30 is from a document on 'proper' composite repairs document, it suggested 1:30 or even 1:60, which makes sense if you actually trying to 'real' repair, that you might want in a say an aeroplane..., where you are actually trying to lap the fibre layers.
so a 5mm sandwich would need 15cm either side, maybe giving you a couple cm of where the fibres lap.
I am sure I am over thinking it...
My motivation it trying to understand it a little better.
From what I understand the board lady approach is essentially making a strong cap (the new laminate), that has a tiny, tiny bit of overlap on the bottom fibre layer, and a lot of overlap on the top fibre layer.
Which as you point out, seems to work very well!
In my case its cracks on the deck, on a very light board, where I believe it would be the bottom fibre layer that failed first, probably just from a heavy foot. Hence my question on how easy is it to get to the bottom layer of fibres.
Go half way down this page. No overlap at all nearly a year later no problems
www.seabreeze.com.au/forums/Windsurfing/Wave-sailing/Whats-Mike-up-to-now?page=2
In the past I've set my router just short of the PVC depth then sanded out the last bit of PVC down to the glass, later new glass over this. I agree that you maybe over thinking this.
If the inside glass broke it's cause the PVC crumbled first because the area was not stiff enough. The whole area started flexing more and more the PVC failed then the glass failed.
Select to expand quoteR1DER said..
In the past I've set my router just short of the PVC depth then sanded out the last bit of PVC down to the glass, later new glass over this.
thanks, that's exactly what I thought of doing, to minimise the repair area. (but thought I would ask first)
Regarding the board repair in your thread, with no overlap on the bottom fibre layer, not arguing that it's doesn't work well. Just trying to understand how it works. It seems to me it works because the stress on the board is spread out (especially the tension from a jump), allowing the foam core to do its job, and any weakness at the joint no longer matters.
Yes the force is spread out over the whole sheet I put in plus the joins are not in areas where a board usually breaks. I think the bottom breaks under compression. I saw a guy do it this way 15 years ago and it worked for him, but I didn't think it was a good way so never did it I've always routed out a 30mm wide channel but on this board the join was to close to the rail so I thought I'd give his method a go.
Update,
I gave the router a go, It was just too difficult to route down to the glass. So I tried one of the little sander attachments, and just sanded until I hit the bottom glass layer, which surprisingly well.
I thought that the bottom layer on this board might be carbon, but its simply glass. Where the EPS is my first attempt with the router. I have another board (same model different size, so I hope to make the repairs a little smaller).
I intend to lap glass over the existing bottom layer, after I tidy things up. Then either some q cells or pvc and a final layer of glass, which I will overlap by feathering the existing carbon layer.
Bottom layer looks like carbon, why do u say glass..?
Then, if you are trying to duplicate the layup exactly, why would you use q-cell mix rather then divinycell ?
(Anyyywayyyy - such a small repair is sooo inconsequential as to make these questions rather moot points. I assumed you meant a major repair in a stressed area. That is so small that if filled with resin and filler, then 4 x 4oz over the top, it would most likely be just fine.
If it is not being thrashed, I doubt it would fail before the rest of the board.)
Select to expand quoteMark _australia said..
Bottom layer looks like carbon, why do u say glass..?
Then, if you are trying to duplicate the layup exactly, why would you use q-cell mix rather then divinycell ?
(Anyyywayyyy - such a small repair is sooo inconsequential as to make these questions rather moot points. I assumed you meant a major repair in a stressed area. That is so small that if filled with resin and filler, then 4 x 4oz over the top, it would most likely be just fine.
If it is not being thrashed, I doubt it would fail before the rest of the board.)
No it's glass, the black in the photo is just dust.
This is the simplest repair, I have a larger version of the board that will need more work. The decks have failed in normal use. (Heavy foot or heel) Hence the desire to recreate a structure that is at least as good as the one before. Hopefully a little better...
These boards are so easily damaged compared with rest of the boards I own.
Will definitely use pvc for the larger repair.
Select to expand quoteracerX said..Mark _australia said..
Bottom layer looks like carbon, why do u say glass..?
Then, if you are trying to duplicate the layup exactly, why would you use q-cell mix rather then divinycell ?
(Anyyywayyyy - such a small repair is sooo inconsequential as to make these questions rather moot points. I assumed you meant a major repair in a stressed area. That is so small that if filled with resin and filler, then 4 x 4oz over the top, it would most likely be just fine.
If it is not being thrashed, I doubt it would fail before the rest of the board.)
No it's glass, the black in the photo is just dust.
This is the simplest repair, I have a larger version of the board that will need more work. The decks have failed in normal use. (Heavy foot or heel) Hence the desire to recreate a structure that is at least as good as the one before. Hopefully a little better...
These boards are so easily damaged compared with rest of the boards I own.
Will definitely use pvc for the larger repair.
save some of the carbon dust and add it to some resin with airosil and soak some balsa (from Bunnings) wait till the resin gels(tacky) hold the balsa down with masking tape
I'm a bit concerned by your pic, it looks similar to the repair I've got in my shed at the moment. There appears to be cracks in the sandwich going further into the board than you have routed out. I can't make up my mind whether I should follow the cracks and remove the sandwich, or just repair where the damage had come through to the surface.
Following that crack could open pandoras box 
,but you have to do it.
I feel your pain.
The crack in the glass is from me trying to assess the damage. (I have got three of these boards, the biggest is fine, the one is the photo has the simplest repair)
What really surprised me is how thin the inner glass layer is in comparison to the outer carbon layer. I guess it looks better with carbon on the outside... I am guessing the carbon layer on the deck takes the load in tension in normal sailing.
I also thought the main function of the inner layer of glass was to the stop board cracking from a depression, as the inner layer will be in tension or is there more to it?
Select to expand quoteracerX said..
What really surprised me is how thin the inner glass layer is in comparison to the outer carbon layer. I guess it looks better with carbon on the outside... I am guessing the carbon layer on the deck takes the load in tension in normal sailing.
I also thought the main function of the inner layer of glass was to the stop board cracking from a depression, as the inner layer will be in tension or is there more to it?
This brings us back to the complex issue of laminating carbon with glass.
About a page back i posted a video about this subject named " Myths about adding carbon to strengthen fiberglass "
It was very interesting and a lot of peoples different ideas about this subject and how it related to windsurfers.
Some say it can be good , others not so much , me.....im still not sure.
Select to expand quoteracerX said..
What really surprised me is how thin the inner glass layer is in comparison to the outer carbon layer. I guess it looks better with carbon on the outside... I am guessing the carbon layer on the deck takes the load in tension in normal sailing.
I also thought the main function of the inner layer of glass was to the stop board cracking from a depression, as the inner layer will be in tension or is there more to it?
Typically the inner glass is one layer of 4oz and the outside 2 layers of 4oz. This makes sense as the outer layer will have the tighter curve in a depression scenario, and has also to cope with impact loads. I'm not in favour of carbon on the outside layer, except for patches under the heels at 45deg to the glass. Carbon has very poor impact strength. Because it's so stiff all the load is borne by the fibers that are impacted, which will snap, then spring back into place once impactor has gone. There will be very little sign of damage, but it will be there. Glass on the other hand will stretch allowing neighbouring fibres to share the load. After the impact any damage will be clearly visible.
Select to expand quoteImax1 said..
This brings us back to the complex issue of laminating carbon with glass.
About a page back i posted a video about this subject named " Myths about adding carbon to strengthen fiberglass "
It was very interesting and a lot of peoples different ideas about this subject and how it related to windsurfers.
Some say it can be good , others not so much , me.....im still not sure.
I think it's a different situation when the carbon and glass are separated by the sandwich foam. They are handling different loads. When one is in compression the other is in tension
^^^ +1
That video was very basic and about slapping carbon on a simple glass layup. Not sandwich construction as we have.
Normally I am seeing approx 2oz under the sandwich though (not 4oz)
Select to expand quoteMark _australia said..
Normally I am seeing approx 2oz under the sandwich though (not 4oz)
Could well be Mark, I find it very hard to gauge the weight once it's been squashed under the d-cell.
Select to expand quotedecrepit said..racerX said..
What really surprised me is how thin the inner glass layer is in comparison to the outer carbon layer. I guess it looks better with carbon on the outside... I am guessing the carbon layer on the deck takes the load in tension in normal sailing.
I also thought the main function of the inner layer of glass was to the stop board cracking from a depression, as the inner layer will be in tension or is there more to it?
Typically the inner glass is one layer of 4oz and the outside 2 layers of 4oz. This makes sense as the outer layer will have the tighter curve in a depression scenario, and has also to cope with impact loads. I'm not in favour of carbon on the outside layer, except for patches under the heels at 45deg to the glass. Carbon has very poor impact strength. Because it's so stiff all the load is borne by the fibers that are impacted, which will snap, then spring back into place once impactor has gone. There will be very little sign of damage, but it will be there. Glass on the other hand will stretch allowing neighbouring fibres to share the load. After the impact any damage will be clearly visible.
So what is purpose of the inner layer of glass? I had thought it was to provide strength when it is placed in tension, but maybe not.
Maybe the clue is that it is actually an intermediate layer between the full sandwich of fibre/pvc/fibre/EPS/fibre/pvc/fibre? Maybe its just there to help maintain the bond with the pvc to the EPS? or just something rigid to hold the shape in the construction process, or maybe it is just away to transmit the the 'rigidty' of the carbon layer to the EPS.
Anyway something does not quite add up.
Select to expand quoteracerX said.
So what is purpose of the inner layer of glass? I had thought it was to provide strength when it is placed in tension,
Yes, that's my understanding, the sandwich is a type of beam, the outside cloth layers provide tensional strength and the internal high density foam compressive strength.
3mm D-cell with no cloth bonded to it is quite flexible. But once it's got cloth each side it becomes much stiffer. So it can then spread compression load over a big area of soft styro foam.
Select to expand quotedecrepit said..racerX said.
So what is purpose of the inner layer of glass? I had thought it was to provide strength when it is placed in tension,
Yes, that's my understanding, the sandwich is a type of beam, the outside cloth layers provide tensional strength and the internal high density foam compressive strength.
Which is what I thought, but at least for this board, I no longer think that is the case. The glass layer is tiny, and very fine, maybe 2oz, carbon is substantial in comparison.
Basically I am suggesting it serves a different purpose as an intermediate layer, and that overlapping of this fibre layer does not actually matter.
Or to put it another way, if the board was hollow the glass, would not add anything structuraly, it would need to be much thicker.
Yes, but it's not hollow, there's some soft stryro foam there. This is load supporting as long as the load is over a big enough area. That's the purpose of the sandwich to spread the load over a big enough area.
And although the carbon on the outside makes the sandwich very stiff where the carbon is in tension, it doesn't do as much in the opposite direction. The 2oz cloth will still improve stiffness in that direction.
Any way, if it was me, I'd still do the overlap, just in case.
Select to expand quotedecrepit said..
Yes, but it's not hollow, there's some soft stryro foam there. This is load supporting as long as the load is over a big enough area. That's the purpose of the sandwich to spread the load over a big enough area.
And although the carbon on the outside makes the sandwich very stiff where the carbon is in tension, it doesn't do as much in the opposite direction. The 2oz cloth will still improve stiffness in that direction.
Any way, if it was me, I'd still do the overlap, just in case.
I am not arguing the glass is not important, just that this inner glass serves to transmit the force from the thicker Carbon, through the various layers, pvc,EPS onto the opposing outer thicker carbon layer. So perhaps break in the fibres in this inner layer doesn't matter.
Though still not sure either way.
Select to expand quoteracerX said..
just that this inner glass serves to transmit the force from the thicker Carbon, through the various layers, pvc,EPS onto the opposing outer thicker carbon layer. So perhaps break in the fibres in this inner layer doesn't matter.
Though still not sure either way.
Hmmmm, only one way to find out. Make up a test sample, same thickness of pvc and carbon with a butt join in the glass on the other side, bend the sandwich to destruction with the glass in tension, (carbon side concave, glass side convex). I bet it breaks at the butt joint.
Select to expand quotedecrepit said..racerX said..
just that this inner glass serves to transmit the force from the thicker Carbon, through the various layers, pvc,EPS onto the opposing outer thicker carbon layer. So perhaps break in the fibres in this inner layer doesn't matter.
Though still not sure either way.
Hmmmm, only one way to find out. Make up a test sample, same thickness of pvc and carbon with a butt join in the glass on the other side, bend the sandwich to destruction with the glass in tension, (carbon side concave, glass side convex). I bet it breaks at the butt joint.
Nothing like a real experiment, but I not sure it would actually answer the question. Unless we measure the strain. 
For example if it cracked along where the glass butted together, but the breaking strength was hardly changed. Then I would say that the butting does not effect the strength substantially.
Anyway something to research while I make sure the board is dry.
Select to expand quoteracerX said..
Update,
I gave the router a go, It was just too difficult to route down to the glass. So I tried one of the little sander attachments, and just sanded until I hit the bottom glass layer, which surprisingly well.
I thought that the bottom layer on this board might be carbon, but its simply glass. Where the EPS is my first attempt with the router. I have another board (same model different size, so I hope to make the repairs a little smaller).
I intend to lap glass over the existing bottom layer, after I tidy things up. Then either some q cells or pvc and a final layer of glass, which I will overlap by feathering the existing carbon layer.
I think you might have a fungal infection on your finger.
