How can I permanently write on HDPE plastic? So I just spent the better part of an hour graduating a food grade HDPE 5 gallon bucket with permanent marker so I can easily fill it with water of a certain amount for homebrewing.

I'm all done. The only problem is that permanent marker wipes off of HDPE quite easily. I guess I should have tested that first...

Anyways... what CAN I use to write on HDPE plastic? Or will I be forced to etch the markings in with a nail or something?

I tried a Google search for "How to write on HDPE plastic?" and came up empty.
From experience, one good way to permanently mark on HDPE is a paint marker. The following is just the first one I found online; no endorsement intended:

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&nbsp;&nbsp;&nbsp;Promotional image from uline.com

They tend to be stinky and a tad messy, but they usually leave clean, visible lines and are basically indelible, requiring a lot of scrubbing with alcohol, acetone, etc. to remove. No idea how well it'll survive power washing, though.