Filling the spar recess

With a loose piece of sandpaper thoroughly roughen the slight depression over the spar.

Mix small batches of micro (suggest 200g resin; 56g fast hardener; 77g micro) at a time and spline into the spar hollow using a steel ruler or similiar flexible item. With some care the profile can be almost perfectly matched to the wing and little sanding will be required in the next step.