Are you opposed to working on a copy of your original texture, if you are not already? That way you can simply select the face is the surface tab and use your face paint texture in the diffuse color slot and not have bump active on just the face.

If that doesn't fulfill your criteria, I'd also agree on making a dedicated bump or even a normal map from your painted face texture. The normal map will read nice and smooth if you use your face painted texture as the reference base, rather than the original.

 

Add: I've not done my own face paint texture map for skin, but there's several I've experimented with from Hero Morph. There's a Harley Quinn face texture there and a joker if memory serves. Both produced a fairly 'filled in' look that is very smooth.

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