The old fashioned way was to 'glove it.' Or rag it :-)

Mix up some DILUTE zinc chromate.

Put on a pair of rubber gloves.

Put on a pair of COTTON gloves.

Pour some ZC into your palm.

Rub it on the airframe.

Or you can just grab a rag with something that will keep the ZC off
your hands, pour a little ZC onto the rag and rub away.

You end up with a THIN coating of ZC - you can see right through it.
Cover the airframe with a tarp, it'll stay rust free for a year or
more... at which time you give it another rub-down. Handy thing is
that you can grasp the whole tube, wipe ZC on all sides at once. LOTS
faster than spraying and a lot thinner than brushing.

Start on one end of the fuselage, work your way down, arouind and back
to the starting point, takes about 20 minutes for a Knight Twister,
mebbe three-quarters of an hour for a Cub.

You do the same thing with pieces of tubing & flat-stock in the shop,
although you usually use 5W motor oil (ie, 3-in-1, etc.).

-R.S.Hoover