Question for you master boilers - boil crawfish or crabs first?

I've always purged my crawfish in salt water for about 30 seconds and then washed them down good with fresh water. That's the way everyone I know does it. They get sluggish for sure, but the mud gets out of them. When I re-wash them with fresh water, it's full of mud even if I had washed them before I purged them which I usually do.