Duplicate the layer you want to smooth
Apply a Gaussian blur to your duplicated layer...
... Until all the details are smooth enough.
Select your duplicated layer
Click on this icon
Opacity mask added. And voila !
If you’re not familiar with opacity masks :
White = 100% opacity
Black = 0% opacity
In the case of our super smooth skin it’ll be easier to “add” smooth skin than bring the details back so...
While selecting your opacity mask, either press CTRL + I or fill your mask with black.
Our blurred layer is now fully transparent.
Select a big, round, huge, enormous brush
Paint with a white brush on your blurred, black and transparent layer to reveal the smooth skin.
Tip 1 : While painting ( with a mouse )… I always start with a big brush and gradually decrease its size to smaller and smaller brushes to work on the details. Big brush : Big surfaces. Smaller brush : Details.
Tip 2 : Try to not paint on the edges of the surface you're revealing. if you touch the edges ... You will be smoothing everything !
Tip 3 : A fine and precise masking work would take hours but if you’re aiming for “good enough”, with just a few minutes of work, you can get to this :
So. As your experience builds with painting, aim for "good enough" instead of "Perfect".
To illustrate the previous tips, a "good enough" opacity mask should look something like this :