Put a new layer over your image, and use the brush to paint it, using the original layer as a guide.
You can get close to this look with noise reduction in the camera raw filter. Take the luminosity noise reduction to 100% and reduce the details.
You can also try the dust and scratches filter (not part of the camera raw filter). A small setting like 2 or 3 is probably all you need.
The results you get from either of these methods will depend on the image.
Cool effect! When I see this I think of MANY different ways to do it. My first approach would be to combine filters in the Filter Gallery. 2nd approach would be to create a blank layer above your image and use the Mixer Brush to create this effect. 3rd approach would be to use the Oil Paint filter and then smooth out your results with Mixer Brush, Smudge or Blur tools.
You will get different answers for this question so try the ones that make sense to you first. A little bit of trial and error will get you there.
Hope you find this helpful!