In order to completely waterproof some basic electronic circuits, I've often used potting epoxy to stop anything getting to the internals.
However, this has mainly been with stuff using PDIP, which is must more robust than the smaller packages the chips on the Pi use.
Can I safely use this approach with the Pi, or will it likely destroy it? Has anyone tried?
What I would consider when doing this is the ventilation and heat dissipation.
Although I've already specified in another answer that there are no ventilation requirements for the unit, it is encouraged that at least 2mm be left from the surface of the board and the housing.
I would assume that this precaution is primarily for the GPU, as that is the component that reaches the highest temperatures during high intensity tasks.
You can do this but I see the following problems you will face: - The connectors on RaPi are not water proof and can not be waterproof in an easy way - You can solve the heat problem by gluing a small heatsink to the CPU, and use an epoxy that has good thermal dissipation characteristics
But this is going to be a VERY difficult task and you may need a lot of units before you'll get this to work properly.
Your best bet is to find a water proof box and then use small cables between the RaPi and the box something similar to this: http://factory.dhgate.com/computer-cases-towers/fanless-waterproof-industrial-single-board-3.5-sbc-car-pc-case-ip67-p39010795.html
Yes it's possible. Cooling will not be a problem, in fact, most potting compounds have higher thermal conductivity than air (e.g. this special compound outperforms air by a factor of 30, 10 being more common for cheap compounds such as silicone or butalene rubber). There are however a few things to watch out:
Curing by-products. Compounds which are not designed specifically for use with electronics can emit water (bad), volatile acids (even worse) or other polar solvents as they cure, which will quickly destroy the electronics you potted.
Curing / thermal expansion. Most compounds will expand/shrink as they solidify. This is mostly harmless for soldered boards, but if they get into connectors such as USB socket or SD card slot, they can lift the connector pins away from the pads. You should prevent the compound from getting into connectors, otherwise you may end up with connectivity issues later on. In most cases, putting adhesive tape around the gaps before potting is enough.
In the end, getting a waterproof case may be a better solution than potting.
You can buy small aluminium plates and glue them carefully with a super-conductive glue to the chips. Then you can plug cables to all the connectors and glue the whole device into epoxy. Epoxy itself is dielectric so you should be safe. However, you can still encounter several problems:
The heat/cooling and stabilisation of epoxy can damage some parts of the device. The pressure changes in that material can be very high. (This can be solved by covering it in epoxy in several steps, lowering the heat produced and the pressures involved.)
The epoxy can get between the cable pins and the plug pins, making them disconnect. (This can be solved by using some super-conductive sprays that exist and that are used in extreme conditions, I would as well invest money into expensive gold-covered cables)
If you seek for waterproofness, epoxy might not be the best option, it is hard but fragile and can leak water though the crack. I would go for some sort of rubber, e.g. fluid latex, which should work well and stays slightly elastic. Either way (epoxy/latex) you'll struggle with problems of water leakage along the cables.
