Paint

[errsemr]

What kind of paint is most suitable for plywood that may become wet?

[RCModelReviews]

Is this for a boat (will it be immersed) or are you just expecting some occasional exposure to water?

Is weight important?

[errsemr]

Is this for a boat (will it be immersed) or are you just expecting some occasional exposure to water?

Is weight important?

The plywood is inside. Plywood is used for bulkheads, servo mount, reinforcements etc. I.e. ideally it should never get wet, but accidents happen. An accident could be a leak or a capsize. If water enters the hull I suppose I will try to bring the boat to land as quickly as practical to let everything dry.

Weight counts as every gram creates extra drag. However, painted wood is lighter than wet wood...

[RCModelReviews]

When I want to water/fuel-proof plywood internally, I tend to use thinned epoxy resin because it's fairly cheap and really is impervious to most liquids.

You can thin it with the manufacturer's thinners although most brands also thin quite nicely with straight methanol.

I mix a small pot (resin/hardner) as usual and then thin it to a consistency that allows it to be easily painted on with a brush. If you thin it too much you might have to give it a second coat but generally, one is enough.