I regularly shoot in the swamps of Florida and use repellents products most of the year. Mesh headnets and such are great until you put your viewfinder up to your face...so I don't use them.
When bugs aren't too bad (windy days and Jan-Mar), I use Repel lemon eucalyptus spray, but it doesn't last long. It doesn't eat plastic, at least. I normally use the Sportsman formula of Deep Woods Off! or the 100% DEET when things are unbearable or I'm in a static position. The Sportsman formula works really well with gnats and I soak my hat in addition to putting it on my face and other exposed skin. The clothes & hats with Permithrin work well for mosquitoes but don't do anything for gnats. The key to using DEET is to have a towel handy to wipe your hands off - otherwise you'll be leaving permanently etched fingerprints on your shutter and other plastic parts as I have done
My only issue is with biting flies (deer flies, yellow flies, black flies). They are huge miserable devils from hell and don't care what you wear or spray on yourself. They are big, undeterred by the wind, and take chunks of skin out of you that leave scars and can get infected. The only solution is to run like hell or wear strips of sticky paper on your clothes and hats, but neither work well and they love to dive bomb your face after circling you like sharks. If I ever figure out a solution to them, I'll let you know