I imagine this would depend a lot on the medium you're using; I've always used the spray-on type for pastels and charcoal and have never had any problems. I'd imagine that oil paints would be fine with a brush-on, but something like watercolors might have some issues.
You cannot use a brush. Wear disposable gloves. Take clean white cotton cloth (a bandage cloth better). Wrap the bandage cloth on to your fore finger so as to make a small ball firmly griped. Take varnish in a bowl ( small) Dip your finger in the bowl. Now slowly apply varnish on your paint like you are touching a baby's face. Do not rub or use or forward & backward movements. Be quick. That's it. ( how I do.) See my paintings in my Hub
Varnishing step-by-step. Your acrylic painting is complete. As a final step, apply the best varnishing techniques to protect your artwork. Your acrylic painting will be ready to hang or be sold. read more