What's the best/easiest way to fix this? I'm pretty sure it's causing me spin out problems. Thanks for any help with my limited DIY skills!

There is no way in hell , that little dot is causing your spin out problems.
Lean a little more forward.

mix superglue and graphite (grind a pencil), wait until dry then wet sand.
For spinout try and move mast base more forward, harness lines further to the back.

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Imax1 said..
There is no way in hell , that little dot is causing your spin out problems.
Lean a little more forward.

Does seem very unlikely, but fins are very strange, someone can do 40kts on what looks like a completely borked fin, and justa slight change to the leading edge can cause spinout.

Kneead it, would be my advice for a quick easy fix

Thanks for the comments. Although I've been windsurfing for decades, I've mainly been into wavesailing and only recently rediscovered how awesome powered up blasting can be with modern freeride kit and a big sail - Blast 117 plus 6.7m / 7.7m Speedster. So Imax1/powersloshing yes it could well be a technique thing but I've never had a fin release quite as often as this one. Not to mention the weird whistling noise the fin makes above a certain speed!
Will have a go at filling it in and sanding if only to stop me thinking about it!!

Finely sand the branding off the fin. Youd be surprised!!

I don't think it's big enough to affect your sailing at all. However you can fill it with super glue and talc or baking powder. Only use graphite if you want it to look like carbon.

A whistling noise is usually from the trailing edge of the fin. You can sand it square (with fine emery cloth) and then take away a bit from one of the edges.