davidp
different country different techniqes, here i would get brush sweeps (with plastic fins) designed for fire doors
you rout them into the bottom of the door so they are hidden, they are sprung loaded so that as you open the door they are raised off the theshold
our building regs also require a similar mechanism ( class M thresholds ) for wheel chair access where the threshold of the door has to be flush with the floor
on external doors they also have to be weatherproof