Electronics manufacturers that have to cope with this environment varnish their PCBs. The main cost on a production line is the time to mask off the connectors rather than the cost of the varnish. You can buy spray on varnish and it doesn't always need an oven to cure it.
When a really heavy duty protective layer is required, this is called conformal coating, and can be achieved by dipping the PCB into the coating. Again, it's the masking thats the expensive part (time consuming), and thicker coatings may require an oven bake to cure them.
The ability of hot components to dissipate the heat will decrease when they are coated, so then a derating factor applies to power components. This is one of the reasons that industrial electronics is more expensive than consumer electronics.
The main reliability issues you will have are on contacts of connectors and switches. Decide if you'd rather get ruggidised, marine type equipment eg Panasonic Toughbook or just replace things when they fail, at a higher rate than normal. It may well not be worth that much effort to protect consumer grade items which will obselete in a short period anyway.