[Ian C]

The readers have no photos to guide their thinking. My first inclination that fits your description of the problem is air bubbles adhering to the emulsion. These insulate the emulsion during some part of the development, shortening the total development time compared to the surrounding area that wasn’t covered with an air bubble.

Since modern film has such a thin emulsion (usually on the order of 0.0005” to 0.0007”), transferring the film into a tray of water after the developing step instantly dilutes the miniscule amount of retained developer so that development comes to a screeching halt. Therefore, stop bath is not needed for film. Stop bath won’t hurt, but is unnecessary.

The density of the image forms during the development step. Normal fixing and a Permawash bath ought to have no effect on the image density or the formation of any spots of low density you see.

But such spots are exactly what we’d expect to see if air bubbles adhered to the film during some part of the development, especially in the early stage.

I’ve found that soaking every film in a water bath and swishing it about to ensure that the film is fully wetted before beginning the development step generally prevents the formation of air bubbles on film.

My experience is that several potential problems are eliminated by soaking all films: B&W, color negative, and color transparency, for several minutes in tempered water prior to the developing step.

My comments are geared to tray development, but the same ideas apply to sheet films developed in tanks.