I'm presently using this aerosol black underseal with waxoil to repair small areas of underbody corrosion. It's excellent for getting into tight corners then staying there. Should work fine on wood.

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Keep well clear of outer bodywork though. It's very black and very sticky!

Richard

2018 Roadster 3.7
1966 Land Rover S2a 88
2024 Royal Enfield Guerrilla 450
1945 Guzzi Airone