It is recommended to use a varnish roller with fibers that are 10 mm or ⅜ inch thick, as this allows the fibers to absorb a sufficient amount of varnish for even distribution of the product. If the fibers are 5 mm thick, they would not allow enough varnish to be applied, resulting in too quick drying and uneven distribution, leading to a lower quality and less durable finish. On the other hand, a roller with 15 mm fibers would release too much varnish, also creating quality issues. It is important to choose high-quality rollers, even if they are more expensive, as they do not shed fibers during application.