I have used Penetrol with oil enamels, maybe varnish.
It does what it says but I guess I just use whatever thinner the paint calls for these days. They make a product, Flowtrol (I think) that I use with acrylic enamels. That works well too but I wonder if water would do the trick as well.
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Either one, any more than a very small amount will result in less coverage, meaning more coats.