I would take a dremel with a cut off wheel and cut down the depth of the cut off wheel along the trailing edge where the damage is. A slot if you will. Then use carbon, I like around 15lb biax glass , lay up a layer or two a little longer than the slot and about 1-2 inches wide on some wax paper or use what method you like. After the glass has kicked, Trim so it fits into the slot. Mix some epoxy and glue it into the slot. From here you can make up a filler with epoxy, some West graphite powder will make it all black. Shape it up. Now you have a edge than can take some abuse without crushing.

+1

One trick is to lay a piece of plastic that is about the thickness of a ziplock sandwich bag over the repair and squeegee the air and excess resin out. There won't be much sanding to do after it hardens if you put the right amount of fibers in the repair. You would prooly want to tape off the repair area to protect the good part of the project. You can also trim the repair when it is partially hard to save time later on.
polyester won't go off when put over some types of epoxy. You can put pigment in surfboard epoxy, or just spray paint the repair for cosmetic purposes.
Also i have found that the most durable bog is a mix of aerosil, a little bit of finely cut up carbon, and epoxy. Works great on the bottoms and foil tips.
The rule for kevlar is bury it deep. It soaks up water and only forms a mechanical bond whereas carbon forms a chemical bond with the epoxy.

First I like to show you some explanation about how epoxies and polyesters do work:

epoxy and polyesters

The cloth in a laminate will always suck up some water when the surface is damaged. (That's why they fill themselves with resin too)
Glass, carbon, kevlar or whatever, it makes no difference.

If the carbon molecules of the carboncloth really are bonded in the epoxy molecules during the hardening (the polymerisation), like you said, then this should mean less stable (disturbed)epoxy molecules.

The strenght of a laminate is largely determined by the fibers in the cloth you use. The resin only keeps all the fibers together; to be more precise, holds them at their fixed position to each other.
It fills the space between the fibres and fills them and holds them internally.

I used a thick slurry of milled glass fibers and epoxy per Jake. After grinding out the damage the edge I'd created actually cut me...which got me thinking: How sharp should the trailing edge be?

What Jake said, but forget about the plastic .

Make your own chopped fibers by rolling
some 4oz glass up on a +/- 45deg and dicing the end every
1/4" with some SHARP scissors. Mix 30% chopped glass fibers
with 70% WEST SYSTEM #404 High Density Filler, and add epoxy.
http://www.westmarine.com/webapp/wc...ord=404+high+dencity+filler#.Ulv93lNqkuQ

Make the mixture dry enough that it will NOT elongate off
the stirr-stick when pulled up out from the mixing can.

Tape the edges of your damaged aria with the Blue 3-M
painters mask , so when Ya sand down initially , you'll
hit the tape but not destroy it. At this point, i usually
paint on a bit of epoxy before mixing in the filler, then
use a Blue shop-towel to remove almost ALL of it.

Use a bit of 3" wide tape on the bottom side to hold
the mixture in place while it goes off.

SANDING
No need to wear a mask here, i use a File and water, till i
hit the tape, or 40grit wet/dry sandpaper. When Ya hit the tape
then pull it and switch to 100grit wet/dry ; then 180 ...

If you spend more than 1/2 hour labor on this job, then your
thinking Too much and need More Beer !!

Bille

Your gonna Laugh, but my sanding block is 1/8" thick
aluminum , (1" X 2"). i have 3 of them:
1) the exact length of my sandpaper
2) the exact Width of the papar, (i get 3 pieces of
sandpaper,after folding the edges)
3) is 1/2 the length of # 2.

Every time you drop your sanding block on the floor
then sand all 4 sides with 100grit by using one of
the other blocks. Do NOT forget this or your sanding job
will be screwed !!

Bille

What do you clean your files with? I've never found anything that I really liked.

Compressed air and a file brush works well with contact cement.

It's Ruff sanding, so i use a 1/2-round or round file
with a rather Heavy groove ; clean Before
the grooves full, and i do it with More water and a file
brush. It only sticks if you don't clean every very often.
Rotate the file every other pass.

Same goes for the Wet & Dry sandpaper, don't let the grit
get filled or ya got to toss that piece.

Wear Very Wet cotton gloves and use them to keep your
work wet; that also helps to keep your girlfriend happy
if you use your hands a Lot !!

Bille

Why do you all try to invent the wheel again??

Go and visit a coach-worker and see how he repairs dented car parts with all kind of putties. These guys have special files for this work. I have some at home and they do work marvellous with there special profile which is designed to not slib up.

But it's all your punishment because you are using sort of putty for repairing cat's.
That has no constructual strenght at all. When hit again, it will break off.

I know it's a tempting and quick method, but don't do it!!!!!!!!!!!!!!

Hi northsea junkie,
Your linked essay didn't cover the difference between the bonding of epoxy to carbon vs. the epoxy to kevlar.
i am no chemist, but i was told by someone that i trust that the reason kevlar fuzzes up when you sand it is because there is only a mechanical bond, whereas carbon and glass have the mechanical and chemical bond. Also i have been told that the reason epoxy has part A and part B is because the part B isn't really a hardener like polyester has a hardener (also called catalyst). Ploy will go off eventually w/no hardener, but part A of epoxy will never harden on it's own.

Sure a sanded and unfinished surface of carbon will soak a minute amount of water, but exposed kevlar soaks water like sponge, plus you can't get it very smooth like you can with carbon. It's no big deal to put an unfinished board on the water that has exposed carbon or glass, but if there was any exposed kevlar the board would prolly be ruined. Bury the kevlar deep if you insist on using it because if you hit it while sanding you just created another step.

The plastic sheeting trick that i mentioned works really well for laminating areas about the size of a rudder or smaller. You will be amazed at the amount of air that you can squeegee out, and the surface is shinny when you peel the plastic. Also i have found that using clear surfboard epoxy which has uv inhibitors tends to save time in the long run. It's not the strongest epoxy but is good enough for most purposes, and some surfboard epoxies are better than others.
The fibers do most of the work, but there are resins with extremely good properties that will add a lot to the strength of the part.

Let's remember too that most of these repairs are being asked to hold up to compressive loads...the fibers really don't matter that much when repairing a ding on the trailing edge of a dagger board.

You ALMOST got it correct ...

When you place a cotton T-shirt in water, Ya get a WET
cotton T-shirt ; the water actually permeates the cotton
fibers.

When you place a carbon in water, Ya get a WET
carbon ; the water actually permeates the carbon
fibers.

NOT SO with Kevlar,when Ya place Kevlar in water
the water dos NOT permeate the actual fibers.
It's kinda like placing a plactic comb in water, Ya
get water dripping down the sides of the plactic, but
it doesn't soak through.

That's One reason Kevlar doesn't work for Squat for a bond
on Nomex-Honneycomb !

Bille

BTW, the glass fibers in that mix i suggested ...
DOES make it structural.

But on a honneycomb you always have a real problem with bondage of the two exterieur layers, regardless of the sort of laminate.

I'm still very surprised that they use it in boatbuilding. If you have followed the last Volvo ocean races, you certainly have seen the complete delaminated panels on the Imoca boats (all with honeycomb sandwhich core).

We in Holland have tried this honeycomb cores already 20-25 years ago in our surfboardconstructions. As editor of a surfboardmagazine in those days,I have tested prototypes with honeycomb. That was a disaster; they were all jumped to pieces!

It's not a structural repair in the sense that it doesn't see broad tension stresses over that area of the board (like the outer skin at the mid-point of the board would see). Also in the sense that if your repair fails, it will not compromise the structure of the board. High modulus fibers excel in tension as opposed to compression. The trailing edge sees crushing point loads (in compression) on impacts and my point is that placing high modulus fibers in something that sees this kind of focused compressive loads doesn't really add much over using fiberglass or even just fumed silica as a thickening agent.

At BMW we used a body filler that was somewhat flexible for repairing dings and dents. It seems for this application, something a little more flexible would be a little better. Any ideas?

Yes to ALL of that.
I should have added, that the size of
the repair will make a Big difference if i use Glass to
reconnect the skins after the repair. Anything over the
size of 1/4" square and I would use glass.

Glass over a repair, if an 8oz were sufficient, then
i Always use (3) layers of 3oz finish glass because
it has a very tight weave and it's rather thin. the first
layer would be 3' larger than the repaired aria, the second
is 1.5" larger, and the last is 3/4" larger than the repair aria.
Large repairs need a minimum of 2" overlap to all sides
of a repaired aria to reconnect the skins.

Adding a fairing coat of epoxy with West System 410
Microlight is the ideal low-density filler to make the final sanding easy.

I would NOT use Nomex-honneycomb in boat building because
of everything you just stated ; it works rather well
on aircraft because the skin loads are much lower. Usially
the skins will only take twisting moments in an aircraft
wing.

I have a Rigid-Wing Hang glider that is 10 years old, and
i will pass-out before my wing will break ; the core is
Nomex-honeycomb. The stuff is a Bitch to work with if
a repair is needed. I nailed a wing-tip one day and it
took me 10 hours to repair it ; make a mistake and I'll be
tossing my reserve. That Really helps to focus on your
work !!!!

I was Lucky in that the guy who taught me is the same
guy who made one of the three prototypes of the F-35 .
I built a cantalievered rigid wing HG that he designed,
and had the good fortune to have him as my teacher on
the project. After which i was the test pilot because
at one time i was # 4 in the world at ACRO on a HG.

Bille

It's a worthy thought, but I can't think of a material that behaves appropriately outside of a university lab...essentially you are looking for something that deforms, but returns to original shape quickly, and has high fracture toughness...I had a prof working on a self-healing composite structure aimed at aircraft wings: http://m.iopscience.iop.org/0964-1726/22/2/025031

All yours for $1 million. In the mean time, I'll continue putting thin webbing in the back of the trunk, has considerably reduced the number of these repairs, as has avoiding the bottom.