A vertical surface is the biggest challenge in varnishing, but I promise you anyone can learn how to do it perfectly before doing coat twelve.
I pour off the varnish I'll need into a one quart plastic mixing jar and work from that. The varnish can is opened long enough to pour some out and then it’s quickly sealed again. Load up the brush with varnish. As long as varnish doesn't drip off your brush somewhere between the mixing jar and the boat you're fine.
Ideally, you want to varnish one hull side or transom section with each loaded brush. That section should extend from the rail to the boot stripe for an even distance from the wet edge. The size of that section depends on how big of a brush you use and how you load it.
The first set of brush strokes will slop most of the varnish onto the surface. Stroke in one direction: horizontally and away from the wet edge. Start these strokes an inch or two away from the wet edge. Make your first brush stroke at the top and the then move a brush width or a little more down for each following stroke until the brush starts to run out of varnish. Then reverse sides of the brush and unload the other side.
After the second side of the brush starts to runs out, catch your runs with a few horizontal brush strokes. Then go back to the top and make firm horizontal strokes over the same area and in the same direction overlapping the wet edge an inch or two.
Firm brush pressure forces varnish into any cracks and helps level the varnish.
Now do a set of diagonal strokes. Move from the top to bottom while overlapping the wet edge. Use firm strokes here.
Do another set of firm horizontal strokes like before, bottom to top.
Now for the last pass with the brush. Use light brush pressure to feather the wet edge and finish leveling. Again, use horizontal brush strokes in the direction away from the wet edge.
Now you are ready to reload the brush and do the next section.
Work quickly. Each stroke should take one or two seconds. You want to reload the brush and start the next section before you loose the wet edge.
You can thin varnish a lot and it will still work, but I thin varnish only if I'm working from an older can that has thickened a little from the original viscosity. If you want the thickest build and highest gloss, don't thin it out. I swear thinning effects surface tension enough to reduce gloss.
After you do a couple of coats, you will be able to judge how much varnish you are putting on the surface by the look of the varnish and the brush feel. You will learn how to hang that varnish just short of the sagging point. That’s where a full-bodied varnish works your advantage.
Keep an eye out for sags as you work. If you find some sagging, try to brush it out even if you leave some dry brush marks behind. That's a lot easier to deal with on the next coat than sanding out a dried sag.
The Pettit BAK-V-SPAR varnish I use has the longest wet edge of any varnish out there. Summer wet edge time is somewhere around 90 seconds. It typically dries to the touch with two hours during the summer. If I varnish in the morning it cures fast enough to let me sand it later in the day and put on another coat the next morning. Fast-drying varnish reduces the time you have to work it with a brush.
This transom actually leans past the vertical plane. The top edge overhangs the waterline by a couple of inches. As an example, a loaded 3" brush will cover about two and a half feet of this transom from top to boot stripe.
