Ok, so this is probably becoming the definitive bumper repair thread...
The last stuff I put in developed small wrinkly bubbles within a couple of weeks, and before long much of it had come off and the bumpers looked like crap again.
I got motivated this weekend to hopefully fix them once and for all. I sanded the bumpers right back with 400 grit sandpaper, then used wax and grease remover, a bumper specific primer, light sanding again then applied a bumper specific black paint as per the picture below. I found that a single can of wax and grease remover was fine, a single can of primer was fine for 3 coats, but I ended up needing 2 cans of the black paint to get 4 coats.
The surface finish of this product doesn't seem as good as the last product I used (paint mist wasn't as fine as I got from the nozzle of the last product, and it feels a bit rough) but it looks fine and will hopefully hold up.
The last product didn't recommend a primer or stress the importance of wax and grease remover, both of which I now think are responsible for poor adhesion of the last effort.
Here's hoping it stays in tact, because if not I've given up on the black bumper dream.
In short, don't bother with weird two-part products, heat guns, "forever black" dye or single part sprays. Do a proper clean with wax and grease remover, spend the time to mask thoroughly, then use bumper specific primer and final paint. By the time you've bought the products and some sandpaper you're up for probably $85.