I have found that horizontal strokes with the tipping brush to leave the least brush marks. Particularly with Perfection. To find the "correct" amount of thinners for the ambient conditions, use a test piece. I use the side of a gallon can and if you feel ANY drag on the brush you must add a scosh (sp?) more reducer. Be sure to use the correct brushing reducer. The rolling, by another person, should be, as already mentioned, in both directions. No more than 2' square at a time and hurry, hurry, but watch for holidays. If possible, do in the shade or in side, ideally. I have roll and tipped both the hull and cabin top/sides of my 39' trawler twice in the 20 + years I owned her. I had no choice on the direction of the brush strokes on the hull, as I had fake plank lines molded in and besides, it is a more natural motion, apart from anything else.
Good luck,
Ian