I like following the directions
on the can. I've been using various Cetol products for many years and with great results. I do not believe the Cetol instructions recommend thinning this product and I never have done so. One of the reasons to use Cetol is that is easy to use, relatively long lasting, and easy to maintain. Having said that, I also think "properly" applied varnish (i.e, lots of sanding, wood prep, more sanding between the 7+coats needed, expensive brushes, and tricky weather conditions in which to work) looks a lot better than Cetol. But I use Cetol because it "looks pretty good" and lasts. You really don't even have to sand to apply a maintenance coat as the new coat enlivens the old coat.