I have used Finishing Spray Varnish by Bob Ross. It is intended to be used
on wet oil paint so that you can add another coat of paint a lot quicker. I
use it on my own oil pastels and if you spray on a few coats, it works well.
On the back of the can, it says to seal oil finishes, you should
use...Damar or Synvar varnish also. Hope this helps.