There are some special
considerations for projects that will spend the lives out in the weather.
Obviously, a good wood finish is important, but woodscrews can also suffer in
the weather. There are three concerns when deciding on the screws to use:
corrosion resistance, strength, and appearance.
Most woodscrews are made from either ferrous (iron-based) or non-ferrous metal.
Ferrous woodscrews are strong and inexpensive, but they can rust very quickly
when exposed to moisture. Even the moisture in the wood itself is enough to
rust a plain steel screw. For this reason, steel woodscrews are usually covered
with a rust-resistant coating.
Typically, there are a couple of choices in plated woodscrews: brass or zinc,
with zinc being the most common. Either one of these plated screws provides
adequate rust protection -- if they're used on indoor project. The problem with
ordinary plated screws is that the coating is very thin. So it can easily be
scratched or worn off. And when that happens, the screws will quickly begin to
rust. Especially on an outdoor project.
Recently, however, a couple of new rust resistant steel screws have come on the
market. They are highly corrosion resistant, and not much more expensive than
ordinary zinc-plated steel screws. One of these new screws looks almost
identical to a zinc-plated screw. It's made from steel with a coating of
zinc-chromate (like an outdoor deck screw). On top of this is a thin coat of a
clear, rust-resistant material, almost like a see-through raincoat. (I got mine
from the The Woodworker's Store catalog.)
The second newer outdoor screw is also made from steel, but galvanized with
several coats of rustproof metals. (This screw is available through
McFeeley's.)
Both of these rust-resistant screws are rated to withstand a wet environment
(in a moisture chamber with a 5% salt spray solution) for at least 500 hours.
By comparison, ordinary zinc-plated screws are rated to last about 100 hours
before the first red rust appears.
Next week, I'll take a look at solid brass, silicon bronze, and stainless steel
screws.
Go to Tip #64 |
