When should a filler be used before applying a finish coat?

A filler is essential on open grain wood surfaces before applying a finish coat because it helps to fill in the gaps and pores present in the wood, creating a smoother and more even surface for finishing. Open grain woods, such as oak or ash, have a texture that can absorb finish unevenly, leading to a blotchy appearance if not prepared properly. By applying a filler, the pores are sealed, which allows for a more uniform finish and prevents excessive absorption of the finish.

Using a filler on porous wood surfaces is another consideration, but the focus on open grain types highlights the specific challenge they present. Closed grain surfaces, such as maple or cherry, typically do not require filler because their finer grain structure allows finishes to coat smoothly without significant surface imperfections. Therefore, while fillers may be used on various wood types depending on their characteristics, the most critical application is on open grain surfaces to ensure an aesthetically pleasing finish.