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Welcome to the Bunnings Workshop community @phillipc. It's brilliant to have you join us, and many thanks for your question about timber usage.
Typically, it’s not recommended to use untreated timber in any outdoor application, even if it’s under cover. H3 treatment is really the bare minimum for above-ground exterior use. While you could technically build the frame with DAR pine, because it has no protection at all, any moisture that makes its way in will eventually cause it to deteriorate. Even in a sheltered spot, changes in humidity and the occasional bit of dampness will get to it over time.
You could try sealing DAR pine very thoroughly with polyurethane varnish on all sides, including inside any joints, but sooner or later, water will penetrate and the timber will start to break down. H3-treated pine will give you a much longer-lasting result, even if it doesn’t have quite the same clean finish as DAR.
Another option worth looking at is LOSP H3 treated exterior timber. It’s dressed all round and often comes pre-primed, so the surface finish is closer to what you’d want for furniture. You would need to paint it, but it strikes a nice balance between appearance and durability for a project like your table.
Please let me know if you have any questions.
Mitchell
