Adding any varnish or oil or paint without doing all the prep work will be a mess so either leave them be and enjoy them as they are or be prepared to strip them back to the wood, fill/repair/stain as needed and then finish with oil, paint or varnish.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
They look fine in the second set of pictures. If they are chairs intended for use outside (which it looks like) they should be fine in showers, and maybe you could take them in or cover them in bad weather and for the winter.
Doncaster, South Yorkshire. Soil type: sandy, well-drained
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I got 4 for £40 on Facebook marketplace
Worried about rain ruining them or affecting longevity!