The main purpose of grain filler is that they seal the open pores of the wood, which means you use less top coat. And that's not just good for your pocket; using less means a thinner film, which is less likely to be damaged.
It also smooths out small irregularities in the surface, to give a better foundation for the finish and binds together the loose, fibrous material that you can't see but is lurking on the surface. So, at Maderas Barber we recommend the use of sealers for wood to achieve a fine and durable finish. In any case, given the specialized nature of this system it is necessary to study each project individually, don’t hesitate to contact us if you have any questions.