About 15 years ago I was looking closely at some early Pictorialists gum prints that Stephan White had in his gallery in Los Angeles. One of them caught my eye in that the margins had not been trimmed and it was clearly a multi coated gum and yet the registration of each coat was perfect, even though the print was @ 11 x 14 inches. I wondered how this was possible, and then it occurred to me that perhaps the photographer had mounted the print to stabilize it prior to printing. I surmised that perhaps he had used a solvent based glue (shellac, etc.) and mounted it on a piece of inert material (Bakelite or even glass). From here I went on to devise my own method of substrating which I'll describe and then note the benefits of this method.
Cut a piece of inert thin material such as formica, heat resistant plastic, etc. I used circuit board phenolic which I bought at a surplus store. The size should be several inches larger than the paper to be printed on.
You will also need some N3 Bregman pins. These are thin .5 inch by 1.5 inch stainless steel pieces with a 1/16 inch high 1/4 inch round stub protruding from one end. The "N3" is the designation for the ones I used. You will also need some tabs, they used to be called Berkey tabs but there are many manufacturers now. These are mylar 1 inch by 1 inch tabs with sticky stuff on one end and a 1/4 inch hole in the other. The pins and tabs can be obtained from a good printers (ink on paper) supply house, I used to get them from the Seward Co. on Alvarado Street in Los Angeles. You also will need some double backed carpet tape obtainable from any carpet store.
Here's what you do with all this nonsense. (Actually its a lot simpler than it sounds and goes real quick as well.)
First take your paper that you want to print on and dry mount it to the center of the substrate material. It's a little tricky since any humidity in the paper may steam out and cause the paper to form a bubble on the board, a little practice and it'll get easy. Degrease with isopropyl alcohol or other solvent one edge of the board next to the paper. Cut a piece of the carpet tape a little larger than the N3 pin and peel the backing off one side of the tape and stick it to the bottom of the Bregman pin. Trim off excess tape from around the pin with a razor blade. Peel off the other backing and stick it to the degreased part of the substrate next to the paper. Repeat this process with the other pin and stick it a few inches from the other pin on the degreased margin of the board. You now have a dry mounted piece of paper with the two pins mounted to the board on one side. The two round stubs are the heart of the registration system and will mate with the tabs which you will attach to the negative. I place the tabs on the stubs of the N3 pins with the sticky side up with the stub of the pins through the holes in the tabs. The negative is then carefully attached to the sticky stuff with the emulsion side up. When used the negative will be turned over. This is just an easy way to assure that the tabs will fit the registration pins (stubs). You may find that taping an extension to the negative cut from a junk negative or piece of acetate may be needed.
What you now have is a board with a piece of paper dry mounted to it with two round stubs off to one side. Your negative has two tabs sticking out with two round holes to match the stubs. Voila! a registration system.
You can now get away with murder when printing gum. Typically my students would print 8 to 10 coats in a session on several prints. The technique that evolved was to coat being light on pigment and underexposed. Typically an underexposed gum develops quickly, looks beautiful, but just slides off the paper as it continues to develop when out of the water. When substrated, you can shake off much of the water and then dry with a hair dryer, if your power can handle it, you can use two. Keep a pot of warm gelatin handy and size between coats.
Essentially you coat with a thin coat of pigment, develop for 2 to 3 minutes (board and all goes in the water,) shake, dry quickly with a hair dryer. Size and dry between coats. Repeat, building up the pigment as needed. The paper does not curl or buckle and in a way, only one side is wet.
When finished, stick the board and print in a drymount press and get hot. Open press and quickly peel up one corner of the paper and strip off. You may neeed to strip it off in stages if it is large. The print can now be trimmed and remounted to a large sheet of the same paper. The board with the pins mounted can be used over again. I have some boards that are over 10 years old and the pins are still firmly mounted.