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Some questions recently came up on the RV-List about what primers worked best for what materials. I ran a little experiment when I first started building, and I figured I'd take a few pictures of what I found out, and put them up here. Shown below are patches of different types of primer that I sprayed on a steel I-beam about 15 months ago, and these pictures were taken this week. They are all spray can products. The beam is outside, and is exposed to the lousy NJ weather every day. I'm about 90 miles from the coast. The beam was already badly rusted and pitted, so I used a grinder to grind down to the clean metal and sprayed immediately.
In closing, I think primer is mostly a personal choice, and no matter what you use, you will get good results. From what I learned with this experiment, I prime and topcoat ALL steel parts. Aluminum is left with just primer to save weight. The opinions you see here are worth what you paid for them, and your own mileage may vary. |
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Shown here is Tempo Zinc Chromate from a spray can. I sprayed 2 patches. The
top patch is a heavy, opaque coat. There are tiny pinhead traces of rust, but they have
not spread, and I suspect that these were just deep pits that I didn't get the rust out of
to begin with. The lower patch is a very light coat, almost transparent. You can see the
silver of the steel showing through. There is no rust at all. The Zinc Chromate obviously
gave the best protection of all the primers shown here, and even a very light coat seems
to do the job just fine.
Here is plain old Rustoleum Enamel from the hardware store. As you can see,
there are some scratches that rusted. I made the scratches on purpose after about 6 months
to see if it would rust. Note that the rust didn't spread under the paint. There is no
rust anywhere else, even at the edges where I didn't grind all the rust away to begin
with. My only concern about using the rustoleum on steel is that I'm not sure what types
of finishes will stick to it. It also doesn't seem to stick to aluminum nearly as well as
steel.
This is Tempo Zinc Oxide in yellow. It goes on like Zinc Chromate, in a kind
of transparent coating. If you look closely, you can see some small traces of rust coming
through. I know the metal was clean here, so I'm pretty sure this rust formed after it was
sprayed. Not as good as the Chromate, but better then the 988 shown below.
This is Sherwin Williams 988 Self Etching primer. As you can see, it has
rusted through. It's very minor rust, but it was the worst of all the samples. I use this
stuff on my aluminum, because it's easy, and the dust isn't as toxic as the Zinc Chromate.
It also seems to stick to the aluminum much better then the Zinc Chromate. If you use it
on steel, make sure you top coat it. You cannot topcoat it with Rustoleum, as I found out.
The rustoleum doesn't stick to it, and it will eventually flake off. Napa also sells it
under the name "Martin Senior 7220".