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Jim?s explanation is right on and explains just about everything needed to understand the process. Although not needed use the process, some additional info will help explain what happens during the exposure and development process. The exposure and developing process is very similar to how printed wiring boards (PWB) are made. The photo resist is an emulsion of polymer molecules, the exposure to UV light cross-links the molecules into long polymer chains while the ones covered by the mask are not cross-linked. The long polymer chains are more resistant to solvents than the short molecules. In the developing process a solvent washes away the short molecules and leaves the cross-linked ones untouched. When making PWB a copper clad board is covered with resist. A mask with the circuit runs is placed over the board and then exposed to UV light. The board is then developed by using a solvent to remove the unlinked molecules. This is often done in a vapor degreaser. Making the stencil is similar only that the resist is a coating on a piece of silk instead of a copper board. The board is then put in an etching solution to remove the copper except in the areas protected by the resist. A ferric chloride solution is often used to etch the copper.
If you wanted to make the markings on your tools deeper, coating them with resist then etching them might be another way to mark your tools. Where I used to work we used this technique to make stainless steel nameplates. It? s interesting why we had to make nameplates from stainless steel. They were used to replace brass nameplate on navy equipment. It seems that the navy loves to polish everything. The sailors were polishing the nameplates so often the markings on the nameplates were polished away. You don?t have to polish stainless and if you did it took a lot longer to wear off the marking. We also used a similar process to make parts out of very thin sheet metal.
A similar process is used to sandblast patterns onto materials such as glass. The pattern is printed on the glass and then sandblasted. The coating allows only the unprotected areas to be sandblasted.
Jim?s explanation is more to the point, but I thought you might find this interesting.
Scp