[Bill Burk]

I have not done this, but I have heard that a pin or knife-point prick on the film will optically diffract light around that area, leaving a white spot on your print for retouching. I have used a fine point red marker for LF film with some success. As someone mentioned, doing this on smaller formats means bigger areas to retouch when enlarged.

This may work for enlargers with Condensers. But Diffusion enlargers, popular for their ability to make dust disappear, also make this roughed up base disappear.

I have used a pin to rough up the base of my 4x5 negatives so it vanishes under the microscope. The result I get is a halo on the print. From a distance, the spot is gone. But on closer inspection it is easily apparent.

When I spot out with opaque, the printed result is very white and MUST be spotted. However the resulting spotting job is very successful.