Higher altitudes will not necessarily affect the cure of varnish, but there are some things to bear in mind when varnishing. Higher altitude allows for faster evaporation rates, but commonly has cooler temperatures so it can slow the cure slightly. Always keep temperature and humidity in mind, regardless of elevation. Faster evaporation rates mean you may want to add a touch more thinner for build coats than you would at lower elevation. Due to the potentially lower temperatures, you may want to wait a little longer for it to cure as well. Finish coats at high elevations may be a little trickier than low elevations to balance thinning as little as possible for good flow, while retaining the highest gloss and wet edge. This may take some practice, or do a sample piece to test on to find the proper amount of thinning needed.