Gallon markings (and our logo) are done via an electrolytic acid etching process. There seems to be some misconception on a few forums/posts that these are somehow "painted" inside the fermenters and kettles.
For a great article on this process, please check out a recent BYO piece here:
PLEASE NOT THAT USING BAR KEEPER'S FRIEND DIRECTLY ON AN ELECTRICAL ETCH LIKE THIS WILL CAUSE FADING.
This is just a chemical reaction of that compound with the top layers of stainless and does NOT indicate in any way the etchings are "painted" !!! We've seen crazy theories about this - bottom line is these types of etchings are used in cookware all the time and are the result of changes in the top layers of stainless at the atomic level and there is no paint or any substance for that matter applied to the surface of the stainless!