20 minutes? Hmmm.

I haven't done the entire boat (hull and deck in one go) in less that 90 minutes!

Like Rob, I work in 12" sections down either side of the boat, switching sides after every 2 sections.

My system is:
* Wipe down the boat with a tack cloth
* Pour about 1.25" of varnish into a 1 qt yogurt container (through a filter)
* Load the brush (3" foam)
* Slap off the excess in the container
* Apply the varnish on a 12" section with firm horizontal strokes
* Level the varnish in that section with light vertical strokes
* Tip off bubbles with horizontal strokes that barely touch the surface
* Move to the next section or switch sides
* Repeat from "Load the brush"

The time between dipping the brush and moving to the next section is 15 to 30 seconds. This will get varnish on a whole 17' kayak in less than 20 minutes. As soon as I reach the end of the boat, I peel off the masking tape at the sheer. I then leave the shop to avoid raising dust.

Here is a video https://www.youtube.com/watch?v=711AfDTuqvk

I think it would be very hard to get a smooth finish, free of brush marks if it takes you take more than a minute on each section. Your wet edge on the prior section will also have started to dry out so you will get overlap that does not blend out.

Moving quickly takes practice. It is easy to put on too much varnish in a spot, not level it off sufficiently, and get a sag. It is also very easy to miss a spot and get a holiday. You need to be very careful around sharp edges and chines as they can scrape varnish off your brush, causing a drool. If you try to go back to any surface after more than about 45 seconds, you will likely leave a mark that will be visible when the varnish dries.

To me, the brush marks are more visible from a distance than dust.