I bought some Milliput superfine and I've had better success with it. I'm not sure if the Milliput I was using was a bad batch or old, but it's still crumbling even now. So what I've got is a JB Weld base (the regular stuff; after seeing a video review/comparison, that was the clear winner among it's competitors), the nut was then covered in JB Weld, and then Milliput superfine went over the top of that just for added safety. I'm also going to heat up the visor before I screw it in tight, just so there's less pressure on the fixings.