​Most people need to do this at the initial point of a project, before they stain or paint a wood surface for the first time. Larger pored woods, such as oak, soak up stain like a sponge, and a thin coat of wood filler gives a smooth, almost glass-like surface to paint or apply your stain on.

 

You want to use a larger putty knife or a trowel to apply the layer of wood filler, because you have a larger surface area to work on, and the larger tool lets you apply a thinner layer.

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Once it has dried, you can sand the surface down to smooth it out before you apply the layer of paint or stain. Again, make sure the wood filler has completely dried before sanding or applying paint or stain.