So I have a go foil wing that is particularly problematic in terms of getting on the mast.

I have never been able to get it to align with the screw hole. I use it just being held on with the tension of the fit.

I already use a mallet to bash it on and off.

Thoughts on drilling another hole in the wing so that the wing can be screwed in? Or just persist with it being held on by tension?

Elongate the hole or sand the fuselage a bit .

Good video on GoFoil tutorials on how to reinstall the helicoil insert for the screw. Perhaps after you remove the existing helicoil, fill in hole was th resin/ putty and redrill to align with your wing hole, then fit new helicoil in fuselage following the steps given. I bought a M6 set from Jaycar for $29. Good luck.
www.jaycar.com.au/thread-repair-kit-for-m6-threads/p/HP1606

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Piros said..
sand the fuselage a bit .

If you plan to use different wings on the same fuz, better sand the wing hole.

What I did is get a bit of wood smaller that the wing hole (for Gong wings, but they are similar to the Go Foil system), and screw a band of sandpaper around it. It allows you to sand properly the hole along the length needed for the fuz.

I had a bit of trouble getting my GL210 onto the fuse at first.

Instead of sanding I put a healthy coat of Vaseline on it..been good ever since.

I manage to get other wings on no dramas, so I don't want to start modifying the fuselage.

I did about 30mins of filing last night and didn't make much progress - might try wrapping the sandpaper as you suggest, Colas.
May even give the Vaseline a go (and the fact that someone called Clamsmasha is recommending it obviously gives it much more weight)

I sand the inside wing a little to smooth it out and then use the Vaseline.

Go slowly. I sanded the fuselage too much and made it loose and had to add bog back to it!