I have a couple of 32cm fins that have damaged tips, but I have no 28 or 30cm fins...
Going to try cutting them down using the angle grinder attachment I have for my dremel, any tips on how to thin the profile of the foil, or maybe just thin the last 5 or 10cm of the foil?
Tips appreciated, would like to keep it simple.
Try just cutting it off square. Not the same as low speed aircraft I know, but this is what a quick google threw up. What do the fin experts think?
"In lower speed aircraft, the effect of the wingtip shape is less apparent, with only a marginal performance difference between round, square, and Hoerner style tips[1] The slowest speed aircraft, STOL aircraft, may use wingtips to shape airflow for controlability at low airspeeds.Wing tips are also an expression of aircraft design style, so their shape may be influenced by marketing considerations as well as by aerodynamic requirements."
Reboxing it from the top end is probably the best way to go about it.
Best tip is take your time. I cut down a lot of fins and re-foil them. Use a texta to outline what basic shape you want to end up with.
If you have a 32 and want a 30 then only cut 1 1/2cms off the tip.
Using a good quality file make sure you sand each side the same and don't do too much at once.
Constantly check your filing is following the fins original foil and basic shape. Once you have got the desired shape use 800 sandpaper to fair it all and get rid of any nicks. The filing and sanding will account for the last 1/2cm.
Use 1200 paper to make it all smooth. Its best to give the whole fin a good sand with the 1200 as well.
Or if you want a rough fin then ask Keef 
I know a guy who just cut a weedy square and did 40knots on it. Do that first and try it, if it feels draggy reshape it. Just make sure to cut it in the direction of flow, remember the board has a slight nose up attitude.
Select to expand quotekeef said..ka43 said..
Or if you want a rough fin then ask Keef
got enough fins larko , just making blanks so they can be reshaped and boxed at any angle that's required
Keep dremel speed relatively low otherwise it'll burn the g10. Give yourself final size +0.5cm then you can alter tip shape a little & lose a bit as you thin tip.
For shaping ive always used orbital sander to cut down pretty quick then wet & dry by hand, g10 is great as you can easy see if foil isnt right by little wobbles in the lines, few measurements to keep it symmetrical & all is good. If you take more than 4cm it can start to effectively move fin a bit far forward in the box so just gotta think a little bout that.
Larko's on the money. Draw your desired outline, preferably with a pencil so you can play with it 'tll it looks good. I've just used a hack saw, making 4 or 5 straight cuts within about 5mm of the outline I've drawn. Only use a file to smooth the outline. Then sand the rest by hand. Don't use a file for the foil. G10 is super soft and you'll be surprised how quickly you can shape the new foil just with sand paper. A file is much too course and you'll end up shaving off more than you want. Post back and let us know how it goes.
Using a file is good if you need to take a fair bit of the fin off. maybe I should've qualified my post. G10 is soft but if you are taking a lot of material off its quicker and easier. Taking 2, 3, or 4 cm off a fin by sanding will give you RSI. so much easier with a quality file and you can see quickly how you are doing. Im not talking big chunks or going hard, using a file is simply easier and quicker. You need to have an eye for the foil, that's the MOST important aspect. As stated, just take your time.
Fin changing and re=shaping is fun if not a bit messy and itchy but when you make a good one its a sweet feeling. Then you hit a rock garden (Lake George) and its back to sanding 
G10 is soft??????????????? maybe all my files are blunt.
I use a right angle grinder with fresh 80 grit paper, mind you I have been known to overcook it a bit (lapse of concentration, distraction of some sort), and have to rebog the resultant low spot.
Thanks for the tips, what about measuring the foil thickness, or just do it by eye. Will post the results
compare foil shape & softness to other fins will give you fair idea. I do measure to keep symetrical just with verniers etc, the layers help alot here, if you watch the wavey lines in layers you can take it down pretty evenly & have nice even foil. Shape tip to pretty close to outline & thickness then work back up the foil to get it consistent. Shouldnt really take that long, last one i did was 10cm off, new foil & outline took bout an hour. Hardest is if you want to move the wide point of the fin.
Select to expand quoteracerX said..
Thanks for the tips, what about measuring the foil thickness, or just do it by eye. Will post the results
Most hardware shops sell "tilers profile gauges" as below
As mkseven says the cloth layers are a great aid to shaping G10, a light sand of the good section will make them more obvious, then continue their curves when you re-shape, and compare each side.
With the profile gauge you could create an image of the good section then scale that image down to suit the sizes of the new foil section. Or better still if the old tip is still intact, record the shape at several points, so you can recreate it.
When I'm shaping a foil from scratch I usually check every cm or so.
Select to expand quotekeef said..ka43 said..
Or if you want a rough fin then ask Keef
there getting better Larko the flange around the leading edge is solid glass and hot out of the mould tonight
Just spend the time to make the tip similar to your other fins, and you're good to go.
The tip doesn't seem to cause spinout, while the chord and mid span does....which you're not touching.
Finally completed this,
cut the outline with an angle grinder, reshaped the tip using a flat bastard cut file, then a once over with a flat sander, with 80grit, and next job is 320 and 600 grit, and some more careful measurements with a vernier callipers and profile gauge.
Yet to sail them thought...
nice, i do think you should match body of fin to tip, it a very small step & probably doesnt matter but still there & it will look nicer when done final sand. Funny that when you finish you'll have a nicer more accurate fin shape than many g10 fins out of factory.
A quick question regarding you delta Keef.
If properly wetted to the excess fibre strands sand down without getting fuzzy?
Stu, carbon sands real nice, it's kevlar that goes fuzzy, don't ever put kevlar in a fin, some day somebody will regret it.
