Rob, the area that's causing the problem is the roof. I start at the top and do one half of the top and then go around and do the other half. Then I move down and continue to do a section on one half and move around to the other side and complete it .Then I do the whole bottom half. It looks real good until it dries, then I see sections where it's dull
John,
Depending on temperature and humidity, one rarely has enough time to apply a finish as you describe.
I would expect the results you are encountering, due to the application method.
Now that you have the dull spots, before you apply another coat, a very thorough sanding to "level" the surface, or more specifically, the overlapped areas that are much thicker than the rest must be reduced.
Get it all even and smooth (and dust fee of course) then apply your finish.
Start at the bottom front, or bottom rear, and work from outside wall, to outside wall.
Apply enough varnish to cover a foot tall x the width of the trailer, then use a good brush to level the finish.
Work in one long sweep, start at one edge, and don't stop until you reach the other side.
Then turn the brush, and repeat the other direction.
Continue, overlapping each brush stroke by at least 25%.
Done this way, your "leading edge" of fresh varnish is always wet and fresh, and your brush will never "drag" because it encounters a partially set area of varnish.
If you had a divider, a piece of trim or something on top in the middle, it would be easier to start at the top and work your way to the bottom, but if not, the method described above is it...unless you have another person who can help.
Then, you can each start at the top, and work down to opposite ends...
Rob