Here's a Florida based pro varnisher who works in humidity and heat.
Since most varnishing anywhere will be attempted at better weather moments,
Vicente Williams has some very interesting tricks to reveal...
google Varnish Teak
http://varnishteak.blogspot.com
He mentions, but doesn't quote, Rebecca Wittman, who wrote the book on the subject.
Doesn't need to, actually.
He prefers to work outdoors.
My caution is to professionally respect the volatile solvents
that are part & parcel of traditional corporate varnish products we have accepted into our life.
Thinners, reducers, driers, de-greasers can shorten your varnish days
- and bestow upon you, not only lasting gloss,
but quality of living problems.
There are 4 blogs in this collection, all interesting!