No matter how much you pay for garden or patio furniture, the weather will wear it out before you do, and it seems to me that every year one or more chairs have to be replaced for one reason or another. The plastic ones lose their shine or embarrassingly a leg snaps, the wood ones look like they would make good firewood and the metal one's rust, especially the welded area (as I found out to my cost, having been deposited as a giggling mess in slow motion on the ground recently when a metal leg gave way!). But a little care each year would possibly increase the chances of your outdoor furniture lasting another season, and putting them away or covering them will keep them protected until you need them again.
Starting with plastic chairs – you can pick up (literally!) these chairs really cheaply, and they stack, making winter storage easier. Most people know they will only last a few seasons outside, but some cleaning care might help them last longer, especially as not all plastics are recyclable if they become unusable. Hosing down and cleaning with hot water and vinegar is a start. (Don’t use chlorine or bleach as these will eat into the plastic and might leave lighter patches on coloured plastic). If they have that chalky finish - which is actually caused by the plastic starting to degrade - this is a tough problem, and you might be tempted to use steel wool and some elbow grease, but a less aggressive method is a rub down with baking soda on a sponge, and once rinsed and dry, polishing with WD40 or a car wax might help restore the finish.
Wooden furniture is totally different, as different woods need different care plans. My first piece of advice is to consult the care label it came with if you can! Wood tables and chairs will need some care or you will face rot or unsightly discolouration. There are gazillions of ways and products to use if you look on the internet or on the shelves of your local DIY store, but a lick of latex paint over an oil-based paint might be something to consider, as it won’t need a touch up quite so often. If you don’t want to spoil the wood look, you might want to consider a water sealant instead, and if neither of these options appeals, then maybe consider a varnish, which will protect while letting the natural wood shine through. Teak in particular just needs a gentle wash down and once dry, a sealer is recommended to preserve the colour of the wood. These are usually solvent-based and of water-like consistency, containing protection against mould, ultraviolet light and moisture.
Metal chairs are surprisingly quite durable, as most have a protective coating that will help keep the rain from creating rust, and the whole chair can be washed down with a mild soapy water then rinsed (and a little WD40 on the welds might be a help!). If they have a plastic ‘weave’ or a faux wicker seat and back, the soap and water wash will keep them clean, although eventually the sun makes the plastic brittle and it will start to break. Storing them or covering them off-season might help.
Steel is strong and heavy but will rust, expand and weaken, and a good paint job will help preserve it. I love those beautiful old cast iron Victorian tables and chairs, and if you own any, these will need a good wire brushing, sanding and painting, but the effort will be well worth it. Cast aluminium furniture is fast becoming the most popular of outdoor furniture as it doesn’t rust, is durable, reasonably light, versatile and requires little maintenance as it comes with a protective powder coating, which is easy to touch up if it gets chipped.
But for now, if you have outdoor space, enjoy your treasure and maybe make some plans for a renovation project later in the year!