I think it depends on how much you are willing to maintain it and if you are flexible on the types of materials you use. Where you live will have a significant impact as well, if you live near the sea up north then you are going to have a harder time than if you live inland in the south.

BLO works but you will have to maintain it regularly and how regularly depends on the weather.

You can use specific wood types that work well outside; pressure treated timber starts off looking a blue/green colour but fades to a silver and won’t need looking after for a long time. Teak is the classic option for outdoor furniture as it’s naturally resistant to pests and erosion, it’s an exotic hard wood though so not the most sustainable. White oak is resistant to rot as well but isn’t easily available. . Any of these with an annual coat of BLO will probably keep them looking new, if you don’t mind them becoming more rustic then you can leave it for longer.

I’d love to know this too!

Finishing a project for outdoor use is really a bit of a balancing act between looking like wood and looking like plastic. Something like a hard-wax oil (or even just boiled linseed oil) maintains the look of the wood but provides little protection against the elements. On the opposite side, paint will provide fairly good protection… but lets be honest, paint is really only appropriate for plywood.

I finished two pieces of outdoor furniture a few months ago; both were made from Alder. For the finish, I ended up with 2 (maybe 3) coats of matte spar urethane. It gives good protection (it’s originally for marine use), but it doesn’t significantly alter the look of the wood. You can certainly tell it’s a film finish, but (at least to my eye), it doesn’t have the “plastic” look you get from a glossy poly or an epoxy finish. There’s a bit of a shine to it, but it’s fairly minimal, even in direct sunlight. So far, both pieces are holding up well, and they’ve been subjected to a range of temperatures and heavy rain. I hate film finishes, but the next time I build something for outdoor use, I’ll probably use it again.

It’s also worth considering what kind of wood you’re using and how you want the piece to age. Something like cedar is naturally rot-resistant, but it will develop a grey “patina” over time. I have read that Sapelle is very good for outdoor furniture with no finish, but I haven’t used it myself. Personally, I’m not a huge fan of the “weathered” look, but a lot of people really like it. My girlfriend wanted a bird house a few years ago, and I made it from a simple white cedar with no finish whatsoever. It has definitely turned grey, but it’s just as structurally solid as when I first built it.

Recently I tried somethingon my garden beds I used on my hammers for a few years, basically I burned the wood, scrapped off the excess char, rubbed on a coat of boiled linseed oil, burned it again and repeated 2 or 3 times. I was told that was how they made the black stave churches in Europe but have looked into it and it’s not quite what they did. It leaves the wood black obviously but it lasts longer than paint( I’m not planning to touch it again for 5 years and even then likely won’t replace anything), and looks way nicer in my opinion.