I've found the following to work very well for cleaning up lots of small dust spots
- Set view to Actual Pixels/100%
- Go to an area of the image with lots of dust spots
- Choose Filter-Noise-Dust and Scratches
- Set Radius to 1 Threshold to 0
- Increase radius until the spots are gone (don't worry about the blurriness of the image)
- Increase the radius until you start seeing the spots again, then back a bit then click OK.
- Go to the history pallet and click on the box next to the “dust and scratches step.
- Click on the name of the name of the previous history state, which will bring back the detail to the image along with all the dust spots
- Select the History Brush set mode to “Darken” Opacity 100% Flow 100% Brush hardness 15-25% brush size just little bigger then the dust spots you want to clean-up
- Paint over the spots with the history brush
NOTE for larger spots of dust or scratches the spot healing brush and clone stamp are better choices