This is a guide on how to waterproof small electronic devices, specifically PCBs but this will also work on other small electronics.
Step 1: Preparation
You will need a bottle of MG Chemicals Silicon Conformal Coating. This is the best for DIY hobbyists looking to add an extra layer of security on their projects. It is resilient and a single bottle will last a long time.
As of writing, it is $20. I still think it's worth the pay as it will add peace of mind for your projects.
Make sure that your surface of application is very clean and does not have any dirt or grime on it. A great way to clean PCBs is to use a soft cloth or brush with isopropyl alcohol. (Pro Tip: You can also use isopropyl alcohol to clean flux residue on PCBs) It leaves no residue and dries up quickly. Watch out for sensitive electronic components such as barometers or temperature sensors as they could be damaged by any liquid.
Step 2: Application
Take a small amount of the liquid from the bottle with the brush and slowly paint it on the board. Start from the center and work your way around to the edges. Think of this like painting a nail. You should not go too fast when applying this and make sure you put a decently thick first coat. Do not paint any ports as this material will prevent proper electrical connection. (Pro Tip: Plug in the cable to the connector, such as a USB cable, if you want to coat around the port) This product also glows under UV light to allow you to inspect that all the necessary parts were covered.
Step 3: Drying
Just to be on the safe side, I allow the first coat to dry in a small breeze for about 10 minutes. You want this coat to stick so don't poke around on the coating in this time. The coating also shrinks a tiny bit when it's dry. After a couple of times using this product, you will be able to identify when the coating has dried but do not attempt to do that if this is your first time.
Step 4: Finishing Touches
Now that your electronics are coated, you will be much more confident using them in or near water. Remember, extra coating can be applied if desired but do not make it too thick. You can easily make repairs on this coating as it is just silicon. This product is resilient in high heat environments but you can easily solder through the coating and then reapply a new coating.
Enjoy your new water resistant electronic device.
Remember that this is only a coating and I still would not recommend dunking the electronics in water although they will be much safer now that they are coated.