The context is as follows:
..., having inherited a safe family seat on the South Coast at the age of twenty-five. ..., and to entertain his own salt-encrusted and sea-blown constituents to huge spreads of strawberries on the Terrace in summer. (source: The Autobiography of a Cad)
It means blown by the winds that come from the sea. Also the reason why they're "salt-encrusted", because the winds carry salt water.
I would explain it like this:
One of the things that makes someone a writer (at least in many ways of approaching being a writer), is the ability to express poetically things that are not, in their essence, all that poetic. I'm no expert so I don't want to pretend I have a professional's grasp on the issue at hand here, but I do remember learning about synecdoche in high school and having a sort of epiphany from it, and then learning about metonymy many years later and seeing their affinity (along with metaphor and simile), and realizing their broad, significant influence on the total of English.
Now essentially, the writer wants to convey what @Stephane posted. People who live near an ocean suffer (or benefit from... whatever your view might be... I don't have any stats on it but for every 40 year-old, prematurely wrinkled-skinned lady in Virginia Beach you can find a 112 year-old fisherman from Okinawa) from the constant salted atmosphere that is just a meteorological fact of living near the ocean. That - as my excessive depiction proves - does not make for good reading.
So these people are being described more poetically. They are sea-blown (blown by the air that originates in the misty, salt-infused area over the massive ocean), as well as salt-encrusted (as in literally covered with the salt that is carried by that air blown from over the sea).
It's unclear why you would be confused (unless maybe you've never had an extended stay by the ocean), but if it is genuinely confusing then I'd bet that it would be because of an apprehension to absorb how widely that North-South-East-West influence of Metaphor-Simile-Synecdoche-Metonymy has on the whole of English, spoken or written, but especially the written. Especially when you imagine those millions of would-be authors sitting down to write their first novels and suddenly discovering that words they've never spoken in their lives (like "perchance," for but one notorious example) are suddenly the perfect words for their masterpiece.
Obviously the "sea" does not "blow". It's just "poetry".
On a wood building the paint or other finish deteriorates rapidly under the influence of wind-driven, salty spray from the ocean.
Once the paint has deteriorated, the wood will become a silvery gray like driftwood.
It sounds as though the author is comparing the acquaintances or "constituents" to something weather-worn like like a building close to the sea.
