Film scans are just naturally dirty. Even the most carefully-done, liquid-gate, Digital-ICE'd scan has some dust and scratches, especially at the very high resolutions I'm working at these days. The film itself has a certain amount of irreducible noise from grain clumping and crud in the film coatings.
I have to do lots of spotting to get really flaw-free images to work with. Photoshop's Dust & Scratches filter, selectively applied with the History Brush, is easily four times faster than using the Clone tool or the Spot Healing brush. I have a trick to using it that isn't in most of the books.
Figure 1 shows a small section of a dirty scan at 100% scale. To clean it up I used the Dust & Scratches filter settings shown in Figure 2. You want to set the Radius large enough that it wipes out the largest dust specks and does a good job of obliterating scratches. The precise setting isn't very important. The Threshold setting is another matter. Look for a "sweet spot" where most or all of the bad stuff is gone but the film grain isn't completely suppressed. It's OK if it's softened considerably as seen in Figure 3, the result of applying these filter settings to Figure 1.
After applying the filter, I assigned the History Brush to that history state, and reverted to the history state previous to that one (Figure 4). Since I was scanning negatives, I set the brush mode to darken, because 99% of the garbage is going to be light. This is the trick you won't find it most of the books (for slides, set the mode to lighten). Using the brush in lighten or darken mode does far less damage to fine detail and film grain. You can set the brush radius wider and paint over many specks in one stroke without visibly distorting the underlying image. Figure 5 shows the results. What few artifacts are left can be cleared up with the Clone or Healing tool.
This filter won't work well if the speck or scratch is right next to a sharp edge or in an area with lots of fine detail. It also won't work with really faint scratches. For those use the other tools. Most crud, though, will clean up real nicely this way.
A variant of this speeds things up even more if your photograph has large areas of sky with few or no sharp-edged objects. Create a mask to select the sky area and apply the Dust & Scratches filter with a radius that will obliterate the small dust specks but not the large ones. Use a threshold setting which is just at the point where the grain is slightly smoothed out.
That will get rid of about half the spotting you need to do in the sky in one fell swoop and will also smooth out the grain a little bit without being obvious. If there are few clouds (or birds) that have edges that need to retain their sharpness, use the history brush to paint in the state from just before you applied the filter to restore those edges.
Spotting is still a tedious, time-consuming chore, but I'm consuming a lot less time this way.
Featured Comment by Edward Taylor: "I still have thousands of negatives and slides to scan from the 30 or more years I was shooting film. I will probably never get it done. Of course, I only shoot digital now since I believe it is superior to film in almost every way.
"In the many arguments that used to take place about the relative advantages of film over digital was the complaint that so much post-processing was needed on digital, while with film there was none. Of course that was never the case, as film required a lot of tedious work even when printed in the darkroom. Now that film is scanned, I think it requires far more work in post processing than a digital file does.
"The craziest complaint in the digital vs. film debate has to do with 'dust on the sensor.' Although this is a real problem which I hope they solve someday, it pales in comparison to 'dust on film,' which has always been a much more difficult and time-consuming problem. Before scanning and Photoshop, a dusty or scratched negative could be very difficult to salvage at all. I have never had a 'scratched' digital file.
"Thank you for this advice. With all the different techniques available to deal with the dust, blemish, or noise problems, we can now get much cleaner prints."