I've sanded the filler in the groove of the windscreen bow & it turned out well, smooth & flat. My canopy was assembled at the factory and the windscreen bow was also cut there. Unfortunately the bow has been cut too deep & there is a difference in height between it & the canopy of up to 11mm. Ideally, I think, it should be about 5mm to allow 3mm for the windscreen plus 2mm for the adhesive so I need to fill & adjust the bow ends to match the canopy. This will require a lot of work, filling & sanding using the same filler as used to fill the groove. I've made a couple of "moulds" that I'll eventually use to copy the original shape of the ends of the bow.
