Hi @Luke
I use the same wax that people use when they have braces for their teeth. I must admit that I did not know that they used wax until my grandson was prescribed braces from his orthodontist. 
It is not as temperature sensitive as bee’s wax and not as permanent as using varnish.
I use a small pocket knife blade to cut off a tiny amount (not any bigger than a small lentil or maybe half of a rice seed). Then I roll it around between my thumb and index finger (
) to soften it up a little and make it round.
Finally I press it onto the rivet and smash it down so that it just covers the rivet end ot the reed and a tiny bit of the reed plate around the square rivet end of the reed. If you mash it down too much, use the knife blade to scrape off the excess that has been spread out too far (for example onto the swinging part of the reed).
For me that has cured most squealing problems 
Regards,
– Slim