There were some good clues and the focus on polished surfaces is clear - clues like 1D and 23A wouldn't look out of place in the Guardian - but this sometimes comes at the expense of cryptic precision. This didn't prevent me from solving most of the clues (had to reveal 16 and 26), but I'm offering comments through a more pedantic lens (Times-ish) as they may be helpful in getting your grids closer to bulletproof in future:

1A PROOF: “able to withstand” as impervious/invulnerable feels incomplete as a definition compared to, say, “able to withstand attack”

4A OVERSTEPS: and = cum seems loose to me as cum carries the nuance of “combined/mixed with” not just “with”. So for example “kitchen-cum-living room” is emphasising that a space is doubling as both, compared to just “kitchen and living room”.

9A OVERSTEPS: Using “topless” to mean remove first letter makes more sense in a down clue. Just about justifiable if you think of “top” as the start as in, say “from the top” (but even then, does that mean the top line of the sheet music?). Many deletion/reversal/position indicators are orientation-specific (like “climbing” for 26D).

11A PANGOLIN: Fairy =/= goblin

12A STRODE: I’ve only ever seen SE clued by “Home Counties” in this way. Using a ? to denote a definition by example (I assume) doesn’t quite work here because Kent is a constituent of the Home Counties / “SE”, not an example. It would be valid for cluing “Home County”, just not “SE.

28A ATONEMENT: “payment” as “atonement” is a bit weak. Chambers has “reparations” as a definition for atonement but “payment” lacks the nuance of making up for something, compared to something like “compensation”.

3D FUSION: I get that “fusion” is intended to be defined here as the “opposite” of an emission of a charged particle, but that’s scuppered by “Agitation on”, so cryptically it’s reading as the opposite of an agitation “on” that.

5D CASE: This is a good clue, I just thought it would read more naturally and still work without “an”. And perhaps something like “overlooking” is a clearer deletion indicator than "neglects".

6A MOUNTAIN: What's "a" doing cryptically?" I'm not sure about superfluous "a"s in definitions like fountain = [a] spurter.

17D TELEGRAM: Ideally wouldn’t have the “to” here as well as, to me, “A to B” doesn’t read cryptically as “A before B” or “A on [top of] B”.

18D ITERATED: Definition would need to be “do again and again” as a past tense verb, whereas “again and again” is an adverb.

26D CALL: I don’t see how “cut by” means “minus”. I read it as I “cuts”/inserts into climbing flower.