The question of when to apply texture overlays to digital paintings and how is one of those craft details that matters more than it gets discussed.

A texture overlay should add physical quality without competing with the painting's own texture. The danger zone is 15%+ opacity on a high-detail texture — above that it reads as a layer on top of the painting rather than a quality of the painting's surface.

I use two kinds of textures: paper grain (for a traditional media quality) and canvas weave (for a painting quality). They suit different work. Paper grain works with linework-based illustration. Canvas weave works with painterly work.

Application method matters. I multiply the texture overlay at around 8% opacity over the full painting, then add a second overlay in soft light mode at 5% for luminosity variation. Two subtle layers combine without overwhelming. A single layer at 13% looks more obviously applied.

One important note: apply texture as the final step after color adjustments are done. Texture under a curves adjustment layer starts to look unnatural.