It depends what kind of electronic bits you want to waterproof. A simple card with no moving parts and just connecting leads responds well to being dunked in varnish and left to drain and dry. There used to be stuff called "Tropicalizing Laquer" which was a varnish that was applied by brush, but when you read between the lines, it was varnish. Being brushed on, you had more control over where it went, so you could avoid painting things like pots and terminals. Stavros' THOMPSONS might be great if it is non-conductive when it has dried.
Then there is putting stuff in balloons and making sure that the neck is downhill from the rest when mounted.
Testing is a bit of a fraught area - a lot of years ago a couple of field managers who had an office in my building were having a meaningful discussion regarding the merits of waterproofing methods for external wire joins. One who was a developer of such things in his previous post used his method, the other, relying on experience, used his, both on their respective phone lines. To test, the "official" version was rerouted so that the join was outdoors in the Blackpool weather. The experienced one was left indoors but immersed in a bucket of freshly gathered sea water. After about a fortnight, the official one n the weather died. After about three weeks the sea water became over-ripe and by popular demand the test was terminated. The design of the "official" IDC connector was changed to incorporate silicon grease.
So the only real test of waterproofing is to dunk it in something conductive - if it still works, the waterproofing works. This is a test chancing destruction of the prototype if you got it wrong and you are taking the chance that if the prototype method worked, it will probably work as well on the rest (QC method). Of course, you can always try the base method on a representative test piece that can be treated then have pos hooked up to it and neg to a separate plate, and see it it produces bubbles when they are immersed in the electrolyte.
After you've sorted that out, its just a question of stopping the other ends of the wires being exposed to air - damp picked up that way and being wicked up the strands is probably the top cause of black wire corrosion.