Help? How to glitter a photo canvas
The method I use is a clear glaze which also acts as a strong adhesive. And, coincidently, the best product I have found is something called Diamond Glaze which is a type of varnish. It is used predominantly in card &amp; jewellery making.

There are lots of manufacturers and it seems universally available. When it dries it provides a completely clear varnish with virtually no opacity or hue. And it is very strong adhesive. Below is an example of one manufacturer (no connection):



Judikins Diamond Glaze on Amazon.com

As it is water soluble, when wet, you can also add inks or dyes to provide different effects. But once dry, about 2-3 hours, it is exceptionally difficult to remove and provides about as permanent adhesive as you can find.

You do need to be careful when applying to media that is water soluble, as it can bleed. So it is best to test it out first. But I've successfully used it on fully dried acrylic paints without issue.