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Zanshin
I agree. If you are in danger of dryrot because of excess moisture on the insides of lockers and cupboards splinters are the least of your problems.
On my boat, a lot of the plywood was fastened to the hull with a chopper gun, and the entire surface spayed with white resin, then spattered with red. Looks like a bad painter's drop sheet. I painted it all with a decent quality single part urethane.
There is no way that you can seal the wood against ALL moisture penetration, and IMHO you might be better off to allow the wood to breath a bit, and to balance it's moisture content to the atmosphere around it as the seasons change. Paint all that you can reach with a nice light coloured paint, maybe two coats with a sanding to level off the dust bugs that you will inevetably get in a locker. Light will reflect well off the light paint and it will be easier to clean. The paint will slow the absorbtion of moisture, but will not prohibit the release of moisture when the air is more dry then the wood.
With that thought in mind, there would be a case for simple oil based house and trim enamel, since they are linseed oil based. Linseed oil will allow the migration of vapour through the film but will resist the absorbtion of water from outside. Linseed oil has mild antibacterial and antifungal properties as well. You will not need to be concerned about UV resistance so even inerior paint will work. I wouldn't concider latex however for a host of reasons.

Make her pretty and you will lover her more.

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