I use an an Igloo-style cooler for a mash tun, and I have a hard time reading the gallon marks inside it. Has anyone found a good way to mark these that doesn’t come off into the beer?
Make up a dip stick or use another means of measuring as you add water the cooler. The markings in the cooler are likely to be inaccurate, as are most factory markings in fermenters and kettles.
Another option is to do the measuring before adding water to the cooler. When I’m filling my mash tun with water, I gather it in a brew bucket first so I can see how much there is. Then pour it in the tun.
I have always found it curious when doing a collaboration at a commercial brewery that the standard measurement of volume is a marked stick of some kind. Very low tech. Very effective. I finally marked a stick for my home brewery this year.
I was going to suggest something similar to that. Although the markings on the inside of the cooler might not be 100% accurate, they will get you close enough. I would take the Sharpie and color the inside graduations on the cooler with it to make them easier to read. You may have to re-color them after a while, but you will be able to read them more easily. I use this on the outside of my wash buckets (which are old bucket fermenters that the ink has rubbed off of) when measuring out cleaning water to use in my conicals and it works great. I also used a sharpie mark on the inside of the fermentation bucket so that I could measure 6 gallons of wort into the bucket.