Feast Watson Outdoor Furniture Oil is a good option as it is specifically designed for furniture. If you were to use a decking product you might find it leaves staining on clothing as contact with materials shouldn't be a regular occurrence.
For a rusty metal outdoor furnishing, start by sanding the rusted areas back with 120 grit sandpaper to remove the bulk of the rust before coating those areas with Bondall 250ml Ranex Rustbuster Rust Converter. This will bind the rust and prevent it from continuing to corrode the surface.
To paint, apply White Knight Rust Guard Medium Grey Metal Primer Paint to all surfaces, followed by White Knight Rust Guard Paint in your preferred colour. - MitchellMc
I just recently repainted an outdoor mirror frame which had started to rust out. I used a wire brush drill attachment and painted the worst bits with a rust converter. I wiped everything down with sugar soap then used Zinsser Bullseye 123 which is a primer, undercoat and stain blocker. Top coated with semi gloss Dulux Aquanamel. A light sand in between coats and it came up looking great. The colour I used was Dowager Grey, which I think was a Taubmans colour. The Aquanamel will dry quite quickly, but needs to have time to harden. Preferably after painting the final coat, leave in the sun for as long as possible to "bake" it. This might take a few days. Good luck. - kacharash4
Look at a marine grade polyurethane. This will help against the weather but also the amount of tinnies that will be placed upon one's masterpiece. Spray is the way to go. If you don't have a spray gun, then roll and finish off by using a fine grade brush, similar to using gloss enamel. You will probably need at least three coats, sanding lightly in between." – darylhewston
I don't like varnish as it ends up peeling when exposed to the elements. I would use a natural oil. You could use a decking oil, linseed oil or a specialist product like Osmo or Whittle Waxes exterior oil. These can all be applied with a good brush. You would need 2–3 coats with a light sand in between. It does need some maintenance, but in my opinion you retain the natural beauty of the timber with oils. There are some very good hard wax oils which do have a gloss finish. The trick is applying multiple thin coats. The oil penetrates the timber and is not just a coating like varnish. " – She_Skills