I just coat the outside. I found it was unnecessary to coat the inside and that doing so meant I had to wash it out before using it.
I've also figured out that if I go directly to the sink when I'm finished masturbating, and rinse it out by inserting the faucet into the Fleshlight's opening, that I can keep it clean without having to remove the sleeve from the tube more than once a month or so, (and that's just to make sure it's in good shape - so far so good). Doing it this way has reduced cleaning time from 5 minutes to about 1 minute, which is MUCH better as far as I'm concerned!
So I'm hardly using corn starch at all, and I use my Fleshlight (Flight) several times a week.