[Antennas] RE: Tower Refurbishing - inorganic zinc coatings

2002年6月26日 23:50:46 -0400

Tim, N9PUZ, recently took possession of a used tower having 
a few light rust spots. He asked about preferred primers 
and paint for this tower.
Several posters noted the use or organic zinc-rich primers 
and paints. These work better than ordinary paint, but 
they are a far cry from the original hot-dip galvanized 
surface for durability and preventing future rust.
My suggestion is to use the modern inorganic zinc primers. 
These are quite different from the organic zinc paints in 
that they provide excellent rust protection. In fact, the 
Electric Power Research Institute found they provide rust 
protection second only to hot dip galvanizing.
This primer is sold in two-part containers. Typically you 
get a gallon of a partially polymerized solution of ethyl 
silicate and a half-gallon of zinc power. You mix the two 
just before use. The surface is generally prepared by 
light sand blasting. A slight amount of remaining rust 
does not seem to hurt the coating. The surface must be 
damp during application and a high humidity is needed for 
the primer to cure properly.
What happens is that the ethyl silicate chemically reacts 
with both the iron and zinc to form insoluble, tightly 
bonding metal silicates. The remainder of the ethyl 
silicate polymerizes and cross links. The result is an 
electrically conductive surface which binds extremely 
tightly to both the base metal and the zinc powder. It 
protects the steel just as galvanizing does; the zinc 
sacrifices itself to protect the steel beneath. About a 
week after the primer is applied, it should be painted with 
a durable top coat. Urethane paints are optimum here.
The biggest negatives of the inorganic zinc primers are the 
high cost (the last time I priced any, about three years 
ago, it was about 80ドル per 1.5 gallon), and the need for 
protective respirators if the paint is sprayed. The primer 
has a limited shelf life too.
Some of the companies that make these inorganic zinc 
primers are shown below. Note that your local paint store 
may have never heard of these. They are usually sold only 
through the industrial divisions of most paint companies.
If you do choose organic zinc coatings, which are just zinc 
powder suspended in a paint binder, the two-part epoxies 
are far superior to the simpler one-part binders. The 
epoxy binders can be painted over, while many of the one-
part organic zinc sprays do not take top coats well.
 Dr. Barry L. Ornitz WA4VZQ [email protected]

 Inorganic Zinc Primers
Ameron (Dimetcote 9, Dimetcote 21-9); Carboline 
(Carbozinc 11Carbozinc 11VOC, Carbozinc 11HS); Davis 
Paint (P-159); Sherman Williams (Zinc Clad 11HS); 
Benjamin Moore (Carbozinc 11); Cloverdale Paint 
(ClovaZinc 2); Devoe (Catha-Cote); Devoe/Fuller 
(Cathatite).