I think ziploc and similar food packaging makers make thin (disposable) sealable sandwich containers. One could put the arduino inside and run the wires out thru a hole in the side. Seal the wire hole with hot glue. Then put the container in a refrigerator freezer with the lid partly open so it can fill with the very dry air. After a while remove the container with the lid snapped on and then seal the lid edge with hot glue. Prior to sealing the lid all the way, squeeze some air out then finish the seal. The container then should be able to flex as the air density inside is affected by heat and cold.
Or add some dessicant and skip the freezer step.
Hi, cook your desiccant first to drive the moisture out.
If there aren't any cables poking through, than there are in fact truly waterproof containers. Mason jars, soda bottles, etc. If you have a mason jar that didn't have any moisture in it to start with, I really doubt any will get in.
If you do have cables going around, then keeping the water truly out becomes much much harder because, as mentioned before, the water can sneak in between wires in a cable or between the insulator and a conductor. You could try caulking or hot gluing the place where the cables come in, and putting some glue inside the cable itself where it comes in, or just using cable glands, but waterproofing is really hard unless you pressurize the entire enclosure to a few PSI, but that requires an enclosure that can safely handle that, and a source of pressure(If there wasn't tiny leaks, you wouldn't need pressure in the first place)
I think pressurization is a pretty common approach on high end stuff.
I'd say your best bet probably is the hole in the bottom approach combined with sealing cable entries well.
If you have the spare power, and the ability to do it safely, you could add a watt or two of heating elements of light bulbs to prevent condensation.
I have a project running an RGB LED outside my house (lamp post) that has been running fine for two years now. It's mounted inside the post and contained in a box to keep the rain/etc. off it but still has the 1/2" holes in the top and bottom and generally open to the air (the linear LED drivers get pretty toasty). I have not seen any degradation in any of the traces, etc.
If you did want to coat it I would suggest a 2 part "laminating" epoxy if for no other reason than the fact that it's a very handy thing to have around, easy to use, and has many years of shelf life. This is the epoxy that comes in two bottles, one clear and the other yellowish clear. You can thin it down with a little alcohol (denatured alcohol) for a thinner coating. Since it's used in boat building you can be assured it's waterproof and won't crack.
The ARistocraft train guys - who run outdoor trains - recommend rubber cement. On the face of it this sounds OK, but they are protecting connections, not electronics.
Key points I think are:
- Don't let water accumulate (use upside down plastic box with the circuitry up inside it)
- Don't put it in direct sunlight (add a little sunshade if necessary).
- Put a lid on, lead any cables out through the lowest point (mount box at an angle if needed)
- Don't seal it 100%, it has to 'breathe' a little bit.