I've used the Wilton shimmer dust and had the same problem. When I paint on fondant, I like to dilute the dyes with the alcohol and use that as a paint.
NOT lemon Juice!! Lemon extract is your best bet! It has alcohol which dries quickly, but it also has some lemon oil which hlps with the smoothing!
It needs to be PURE lemon extract 9not artificial) and you can get it in clear!
It should be mixed about 50/50 with the powder and try dry-brushing some of the powder on after the wet coat has dried! That'll fill many of the brush marks adn thinner spots!
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