Right, Farmers Reducer might work. Once you use it, you can't go back if you don't like the result. I would try printing with a 0 or 1 contrast filter first, before messing with the film.
I shot a a pic of the sun coming up at this years Balloon fiesta here in Albuquerque. Great image, however.. its a bear cat to do dodging and burning with this image. Heard somplace that you can use Silenium toner to decrease contrast. It's a near perfectly exposed for the conditions that were present.
As others have pointed out, there's no going back... it's best to exhaust all other options before you resort to modifying a (probably unique) negative.
I've gotten several grades below zero by using SLIMT - basically bleaching the paper very weakly post-exposure but pre-development. I got best results by combining that with split-printing so it was still punchy in both highlights and shadows while fitting both on the paper.
Try making a print with all the tones you want, it doesn't have to look like the final result which you are striving for - only to be usable as an interim step, and then re-photograph the print and use that neg for producing future versions of the picture.
Contrast Mask +1 Pros: Will not damage the neg, contrast can be adjusted if you make different contrast mask. Cons: Will have a hard time to register your neg and mask if you don't have register punch and pins. EXTENDS your exposing time, especially if you are over exp/dev and you have a very thick neg.
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