My experience with my most recent crash which involved lots of road rash are that if you use Tegaderm, you must leave a little outlet for fluid to escape so it doesn't build up under the bandage. Tape a piece of gauze over the outlet to catch the fluid that drains out.
Also, there this stuff behind the pharmacy counter that's really expensive but works very well. I forget the name of it though, but I used it for road rash on my butt cheek and it worked extremely well. It came in a 8" or so square and was on the only thing besides standard gauze that was big enough to cover the road rash patch.
EDIT: "Duoderm" is the name of the aformentioned product. OBRA has a
guide to wound care which can be helpful too.
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Cat 2 Track, Cat 3 Road.
"If you’re new enough [to racing] that you would ask such question, then i would hazard a guess that if you just made up a workout that sounded hard to do, and did it, you’d probably get faster." --
the tiniest sprinter