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Hello @Lee777
Because you’ve already used Cabot’s water-based natural decking oil, the timber has been saturated with oil, which is designed to penetrate and repel moisture. Varnishes, on the other hand, form a film on the surface and rely on a clean, absorbent, and oil-free surface to bond properly. If you apply Cabot’s varnish directly over the oiled Jarrah, there is a high chance the varnish will not adhere well, may peel or blister, and generally won’t give you the durable protection you’re aiming for.
If you want a varnished finish, the decking oil would need to be thoroughly removed first. That means sanding back the Jarrah to raw timber and ensuring there is no residual oil. Only then would it be suitable to apply a Cabot’s interior varnish or polyurethane, which will give you that hard, protective film.
If you’re happy with the look of the oil but want extra protection, it’s usually better to continue with maintenance coats of the same decking oil rather than switching to a varnish. Oils are designed to soak in and be replenished, whereas varnishes are designed to seal and build layers, so they are not generally compatible on top of one another.
If you need further assistance, please let us know.
Eric
