This is a very worthwhile technique to protect any electronics that has a chance of getting wet. I have spent weeks repairing electronics that was damaged by water, which would probably have been fine if the manufacturer had spent a little more time on coating the PCBs. Enclosures designed to be waterproof often end up not being so.

The types of most relevance for most things are probably acrylic or silicone. Acrylic generally and silicone if needed for higher temperatures or some chemical resistance. Other types are more resistant to chemicals and heat but probably more expensive and/or harder to use.

Application in small quantities will probably be by brush or spray. A spray can is much quicker and easier unless you have a lot of parts to mask, then maybe quicker to brush around them. Note though that the coating will spread out somewhat and may be drawn into unmasked parts.

• Moving parts: buttons, encoders.
• Connectors. Put the mating parts together first too.
• Test points and programming pads if still needed.
• Antennas or other areas sensitive to dielectric constant. (Unless you design with coating in mind of course).
• Sensor holes - like barometers and humidity sensors.

Parts not conformal coated may be suitable to be protected with silicone grease - particularly connectors.

Acrylic softens in IPA. I read on a forum that tetrahydrofuran (THF) would work much better, but I've not tried it. Supposedly acetone works but might damage the board.

Acrylic and silicone can also be burnt off with a soldering iron.

I have found that I can use multimeter probes on acrylic coated parts without removing the coating - it seems the probes can just push through, but not super reliably.

Acrylic conformal coating looks, feels, and smells very much like clear nail varnish. I don't know how they differ. I have now tried applying some nail varnish to a board. It was thicker than the conformal coat I have, so took a bit more work, and harder to put in small spaces. Finished coat also looks thicker / rougher. I have not tested effectiveness in water.

Other grease coatings. Lanocoat if that's what you have on hand?

Legitimate rather than hacky: epoxy. Maybe not the best choice normally, but on a boat it might be what you have already.

How much does coating affect frequency of crystal oscillators? They are easily pulled by changes in capacitance.

• A good guide to conformal coating
• Electronics Construction / Hardware Notes