Yes, exterior paint is UV resistant, and will protect the plastic ties
(which are supposedly UV resistant, too.) Use spray cans. Any exterior
or rust-proofing brand will do nicely.
But you need not remove the rails - just wipe off the top and inside
edges when done (best done before the paint dries.) You can make this
part of the job easier by wiping the tops of the rails with an oily rag
first. Use several flat (not gloss) colours - dark browns, black, dark
greys - and spray randomly with each colour, not waiting for the coats
to dry. the subtle variations will make both rail and ties look more
realistic. When you join the rials with railjoiners, scrape the paint
off where the rail joiners contact the rail base. Cosmetic emery boards
are good for this, they are small, can be cut to handy shapes, and are
cheap. There are found near nail polish and such in the cosmetics aisle.
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