When you buy garden items they look very nice but after a hard summer and maybe even harder winter they need a little tlc.
Varnished items need the most care. Most of these tend to be ordinary softwood and varnish has been applied as an exterior protection. The problem with varnish is that once it starts to peel you need to act quickly.
Sand down flaking areas and apply at least 2 coats of varnish and then sand and varnish the whole item to restore the protection.
Sawn wood. I.e fencing panels, picket gates fence posts. These tend to have an unsanded sometimes rough surface. Care for these is a little easier as they basically just need to be given a quick brush to remove dirt / debris/ loose material and then painted with a suitable fence paint. This should ideally be done every 2 to 3 years.
Other painted / treated timber. Painted items should be checked each year and washed down with soapy water to remove accumulated dirt, paying special attention to jointed areas where debris can accumulate. Make sure all debris is removed and then repaint if needed.
Oiled / stained woods. This is the easiest to maintain. Woods oiled with Teak oil or Danish oil merely need to be washed down to remove debris, dried and a fresh coat of oil applied. Generally these oils are used on hardwood which is naturally water resistant so this is merely a cosmetic exercise.