On Nov 16, 6:16*pm, Home Guy wrote:
Michael Dobony wrote:
I have an all-in-one AC/furnace sitting outside. The circuit board
was replaced a very short time ago, about two months ago. It is
already showing signs of degradation (rust on a diode lead).
Is there any spray coating I can put on this to prevent further
degradation, assuming it is still in operating condition?

What will probably do the job is something called "conformal coating".

Here's one example:

http://www.mgchemicals.com/products/422a.html

==============
Silicone Conformal Coating

Ideal for high temperature environments. Silicone Conformal Coating
(422A) is a flexible finish product that provides a protective coating
for printed circuit boards against moisture, corrosion, and thermal
shock. It protects and insulates electrical and electronic components
and assemblies, including generators, motors, transformers, relays, and
solenoid coils. For spraying, liquid can be thinned using M.G. Thinner
Cleaner. Thin up to one half part thinner to one part coating. Also
available in a micro tip pen format.
==============

I'd remove the board, clean it by pouring some drug-store alcohol over
it while scrubbing it with a soft brush, let it dry completely and then
apply the coating.

If the board is relatively clean, then maybe just turn off power the the
AC unit and spray the board with the coating without taking it out.

http://tangentsoft.net/elec/movies/tt05.html