I know EXACTLY what you mean.  I was doing something exactly on those lines just yesterday.

It's not trivial, but it's doable.

For hard "spots", you can try the Spot Healing Brush first.

But here's a neat trick I sometimes use that can nicely increase the overall smoothness:

1.  Select just the skin area.

2.  Contract the selection a bit and feather the edge, just to make sure you don't mess up the edges.

3.  Filter - Noise - Median, and adjust the radius to be fairly large.  Watch the image to see the smoothness level.

4.  Edit - Fade and knock back the effect, so that it's not perfect, but makes a subtle improvement.

Then, for gentle darkening lightening, try the Burn and Dodge Tools, set to a low Exposure level (and possibly with Airbrush Mode on).  Work gently.

Cloning with low opacity can sometime help as well.

-Noel

As there is hardly a task that can not also be done with Smart Objects I would recommend …

• Lifting the skin onto a Layer of its own (with a fairly good Selection naturally)

• converting to a Smart Object

• applying a Layer Mask of the current transparency

• applying a big-ish Gaussian Blur to the SO (not the Mask) and either using a noise-Pattern Overlay set to Linear Light and reduced Opacity or the Filter Add Noise

• reducing the Layer’s opacity

Edit: For additional masking a Layer Mask on a Group can be used.

The result will likely not be better than with Noel’s method, but at least the file will be larger …

Sometimes just painting on an additional Layer (with the appropriate Pattern Overlay to fit in with the image’s noise/grain) can help, too.

c.pfaffenbichler wrote:

 

• applying a big-ish Gaussian Blur

Just keep in mind the Median Filter tends to preserve edges better, so it tends to work closer to edges.

-Noel