Peter J. Blackburn illustrates a time when shifting in reverse was the best way to move forward!
The following are some concluding remarks in my report of how gum printing has changed for me 0ver the last several years. Tweaks and Twists: Part 1, can be found here. I pass this information along for your benefit. Glean as you wish and may you experience much success in your own printing endeavors.
As I perceive gum printing to be more in accord with the traditions printmaking rather than painting, my usual order for laying down color (CMY or CMYK) has followed the custom of yellow first, then red, followed by blue and finally, the optional black. Following that tradition has proven to work like a charm much of the time. There have been times, however, when I have encountered frustrating moments when one layer will not remain attached to the previous one during the wash cycle. Sometimes the red will flake off the yellow layer, or the blue pigment will simply rinse down the drain taking all the glorious shadow detail and tonal density with it!
[Gritting teeth.]
What makes this bit of malady even more unnerving is that when printing in the exact same manner the following day, those same rascally pigments work glitch-free. Each layer clinging to and playing nicely with the previous one. Shadows and tonality remaining in place singing a happy tune. Sweet smiles adorning every face while the blue birds dance a jukebox jig on the window ledge.
[Crickets chirp.]

19.5 in. x 11 in.
tricolor gum bichromate photograph, wood, wire
Printing order: blue, red, yellow.
Then on other occasions, those little round spots, er, bubbles, you know— fish eyes magically appear as I brush one layer over top another. Hard as I try, it takes light-years to blend those pesky areas where the mixture will not stick. It’s like a force field has been engaged, repelling all fresh pigment, thwarting any penetration by common brush.
[Thinking to myself, “I wish The Incredibles would find somewhere else to hold their family reunions. And take Violet with you!”]
So I keep brushing… and brushing… and brushing… and brushing, all the while wishing I had Spock’s phaser to vaporize the resistance!
[Sighing cross-eyed.]
Oh, happy days are here again! Seems the remedy for both ailments has been discovered.
In the shouting words of Captain Kirk,
“Reverse course, Mr. Sulu. Print the layers in reverse order. Blue, red, yellow. I repeat: blue, red, yellow!“
With that command, the heap of pigments I tossed aside over the years for dereliction of duty, enough to generate a new galaxy and fill a black hole, suddenly and stunningly worked splendidly. And neither Violet or her trouble-causing force shield have been seen since.
Well, look over there. Blue birds are dancing on the ledge.