The application specs on the product I'm using (Interlux Schooner 96) indicate nothing contrary for this as a working temp.

Every varnish and finish is different. Having gone through a variety of brands and even inter-band products I settled on Z-spar Captain's 1015 varnish. It brushes out very smoothly right out of the can and stays wet longer than others.

Sounds like you're 90% there with technique.

I use a 4" foam brush to lay down varnish quickly in a criss cross pattern (with grain/cross/with) over 12" wide sections keel to sheer line and deck centerline to sheer line.

The hull is the best place to start and learn to put the right amount of varnish on your brush.

Dip your brush into the varnish about 1/4" deep. Most people apply too much varnish and come back later to see drips and runs.

If this were not your final coat, I'd suggest to you and others try using less vanish than you think with each dip of the brush and if the surface has dry spots later, you'll know to add just a bit more to your brush as you dip it into the varnish.

Start your first stroke parallel to the keel line. Work an area about 12" wide. Your first stroke/contact releases the most varnish, going with the grain, continue parallel strokes down to the sheer, then cross the first strokes, reaching back to the keel and pull down the first heavy stroke to even out the coating. Finally go again with the grain from keel to sheer to smooth everything and make sure you didn't miss any spots. I learned this criss cross pattern from a varnish rep who said this method evens out the coating and reduces missed spots. Work over a wet edge and on down the length of the hull. Do the same on the other side of the hull. Any uneven varnish overlap on the keel will not be noticed.

My kayaks all have a sharp angle between the deck and hull at the sheer line making the varnish transition easier to control. Once the hull is varnished I wipe the deck edge at the sheer line with a paper towel to remove any runs.

The un-thinned Captains varnish brushes out more smoothly than others varnishes I've used and stays wet a bit longer allowing spot touch ups within in a couple minutes. So have a lot of bright lights on the boat so you can catch their reflections on the wet surfaces to reveal any missed spots.

Detailed areas are a problem for foam brush use because if you press, the foam brush tip drops a pool of varnish.

Varnishing under the cockpit coaming is tricky to do without runs. On the deck I use a 1" foam brush and coat under the coaming first and then the recessed hatch lips. Go back and remove any drips /runs before coating the deck with the 4" brush. I work down the deck jumping from side to side keeping a wet edge and center line.

Carefully wipe the hull sheer line to catch any runs.

Good luck!
Rob