The problem of moisture ingress in outdoor electronics equipment?

A protective coating seems like the best short term solution but from what I understand the extra cost prohibits it.
I was looking at the coefficient of permeability tables for plastic. Plexiglass which I think is similar to Perspex has the highest permeability to gaseous water. Do you think this is a non-consideration when trying to keep moisture out when there are short periods of vacuum on airtight containers. Not suggesting changing materials but just wondering if it plays a role in moisture migrating into container. Kajunbee

No plastic container is actually gastight. Only hermetic (welded or similarly sealed) metal containers with ceramic or glass feed-through are.

That's why Goretex or similar venting systems are the only way to achieve long term dry conditions in non-hermetic cases.

Kajunbee said:

I was looking at the coefficient of permeability tables for plastic. Plexiglass which I think is similar to Perspex has the highest permeability to gaseous water...

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Pexiglass and perspex are commercial names for acrylic based plastics; they are thermoplastics and melt when heated. Same is true for polythene and polyvinyls (they contain too much of plasticizers).

Croxx-linked polymers are the best; bakelite, epoxy or even XLPE are extremely resistant to moisture.

But it is not the moisture that is the culprit- it is harmless- but liquid water is the danger. Because once corrosion starts the performance degrades.

Did a fair bit of reading on moisture ingress yesterday. One sight said that seals that go through vulcanization process can give off hydrogen sulfide gas if not bonded properly. Mix hydrogen sulfide with water and you have a pretty lethal cocktail. I imagine the seals would have to be quite large for this to pose any sort of a problem. I've seen first hand the effect it has on copper. The town I lived near had a sulphur plant. The sulphur in the air would mix with the moisture on AC evaporator coils and eat them up. The tubing would turn black but there would be no apparent holes in them. But if you sprayed soapy water on it the whole end of the evaporator would foam up. I guess the acid ate up the softer metal in the copper and made it porous like a sponge.

Kajunbee said:

... One sight said that seals that go through vulcanization process can give off hydrogen sulfide gas if not bonded properly. Mix hydrogen sulfide with water and you have a pretty lethal cocktail.....

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All natural rubber use sulphur /sulphur based compounds for vulcanization. Hydrogen sulphide has a strong odour (rotten egg smell) and you can smell it at 1ppm level. Hydrogen sulphide dissolves in water to a small extent but I do not know what you mean by "a pretty lethal cocktail"...

Many modern gaskets use synthetic rubber that do not need vulcanization (neoprene, I think). The problem (sulphur plant) you mention is caused by sulphuric acid; it is formed from the sulphur dioxide (that comes from the plant) and which combines with moisture and oxygen and produces the deadly acid. The acid works on almost every exposed surface- water bodies, cement walls (paint) and base metals.

You can use a soft polythene sheet as the gasket; it will work wonderfully.

I know the rotten egg smell very well. I was completely wiped out by hurricane Katrina so we decided to move off the coast. Where I now live we are on well water. The water has that rotten egg smell you refer too. It has taken me years to you get used to the smell. AWFUL!!

Sulfur dioxide is also emitted from vehicles burning high-sulfur fuels.
Or from power plants burning high-sulfur coal.

And correct, sulfur dioxide will eventually combine with water to form diluted sulfuric acid, the dreaded "acid rain".

This is really the foremost contributor to electronic component failure which are exposed to the environment.

Chip resistors are particularly vulnerable as their silver electrodes, used to bond the resistive ink to the endcap, corrode very easily. A clear sign of sulfur attack is that the resistor goes high impedance.

That is the reason the chip resistor industry offers "sulfur resistant resistors".
Google the term.