How to apply diamond dust to a canvas?
"Diamond dust" refers to glitter than is actually composed of crushed glass instead of the traditional shiny flecks of plastic. This differs from glitter in that it is composed of larger pieces and therefore has more problems with adhesion. My guess, based on the limited information, is that Summer Rose is having a hard time because the glitter is leaving gaps where the adhesion shows through and is too sticky?
To avoid this, there are a couple options.

1. Use a better glue 

For glitter like this that is both heavier and larger, a more robust glue is needed. Instead of spray glue, try using PVA glue or hot glue. If you find these adhesives do not dry completely clear and do not offer the desired effect, there is diamond dust specific glue called Diamond Glaze which seems to be popular among crafters if you'll be using diamond dust frequently.

2. Use traditional glitter instead

Traditional glitter is cheap and super easy to use. It can be adhered with nearly any liquid glue. Just don't accidentally drop it on yourself.

This forum on diamond dust also might be helpful to you where a similar question is asked. It's lengthy, but you might get some good tips outs of it!