Our BROAD RIB ROOFING and STEEL PLATE SHINGLES are not protected with
a coat of richer metal like those made of tin or terne plate, and
consequently their _durability depends upon the paint_ with which they
should be covered.
For this purpose nothing, so far as we know, is equal to a mixture of
_pure unboiled linseed oil_ and the _oxide of iron_. The Shingles should
have two coats, one before they are put on, on both sides (this we do),
the other immediately _after they are laid_. This last coat should be
repeated _every four or five years_.
Remember, it is the _rust-preventing_ qualities of linseed oil, combined
with the oxide of iron, that makes steel or iron sheets resist the
corrosive action of oxygen, which is ever present in the atmosphere. (See
page 101.)