The Dye Sublimation process is amazing. It was invented in Italy for the fashion industry. You can google (Youtube) it and watch the giant roller presses make fabulous fabrics. The dye/ink is actually driven into the cells of the fabric during the heat/pressure phase.
Well, anyway, I first did the process myself (which is lots of labor) one flag at a time, but then we sold so many we had to contract print (by the 100 yard roll) which I do weekly.
The fabrics (many weights) are mostly used for clothing (particularly sports fabric) and can be put in a washing machine!! I have taken flags off the poles and put them through my washer for 10 cycles and they came out perfect. Not that I'm recommending that to you!! LOL, but you can lay it in a sink or on an outdoor table and properly wash and rinse it.
You don't need to spray them with anything but you could if you want to.
My largest customer pool are people who already have an open flag and then order more, like SALE, or different colors for the seasons.
I do custom (LOGO) flags for entire towns and they are constantly adding more. Telluride, Colorado recently did "ART WALK" and had their merchants change out their regular OPEN flags during their art festival week.
They are so classy, and relatively inexpensive compared with other signage.
All of my sub-contractors and suppliers are women owned/managed companies which wasn't on purpose, but is lots of fun.