How Median Blur Helps Painters
Preserves Edges, Removes Noise: It eliminates tiny textures (like pores, individual leaves, or fabric threads) but keeps the hard boundaries between light and shadow.
Creates Paintable Shapes: It breaks a complex scene down into distinct, flat masses of color. These map out exactly where your brushstrokes should go.
Accurate Value Mapping: By removing distracting details, you can see the true underlying value structure (the darks, midtones, and lights) without being fooled by local texture.
I use Affinity's Live Median Blur for this