Yep. Looks like classic Russet mite damage!! You need at least a 30 - 60x mag to see them. Little torpedo shaped fuckers!!
Get Nuk Em or Green Cleaner. They are harder than fuck to kill off completely. Hit them hard and hit them often. But remember to rinse your plants the next day.
I got hit hard last year. I'm a fairly large medicinal grower. 48 plants each on two tax lots.
If you're not being effective with your spraying your buds will begin to brown up.. hemp Russets love buds.
You don't need to focus as much on the top side of the leaf, but blast away on the underside.. they hang out in nooks and crannies on the underside.
I spoke with entomologist here in Oregon at Evergreen Grower Supply. By the time you see damage on a leaf there can be as many as 1-2,000 Russets per leaf!! I started to see them last month again and I sprayed wettable sulphur for three weeks every third day..
Then I spent a week doing a weak foliar feed with cold water.
Last week I switched to Green Cleaner for the plants that I still feel comfortable spraying..
next week I release predator mites. Californicus and Andersoni mites to see be through the end of Sept!
Hope that helps!