Hard Case solution
by Kegler

As indicated by the central entry, METAL PLATING, seven answers get “coated” with the chemical symbol for a metal to become a new word before entry into the grid.
 The added letters are highlighted in the grid.  The seven metals used are chromium (10A), platinum (24A), iron (42A), silver (15D), lead (20D), tin (25D), and gold (34D).

Hard Case solution grid

Across
1SA(C)K (ASK anag)
4O,W + NGOALS (SLOGAN anag)
10CL(ASS)IER
12WHO + A
14CH + ELA (ALE rev)
16RU(PE)E
17SEtABED (rev)
19AMP (MAP anag)
21LIFT (hidden)
22B + ASH
23BIO (1st letters)
24PLU(SHE)StT
26METAL PLATING
27P(H)EASANT
29NOD (rev)
31isSUES
32INTO (TONI anag)
33GAY (odd letters)
35T,E + RED + O
36TRAD + E (rev)
38BO(I)SE
40F + LAW
41NORSE + ME + N (SENOR anag)
42FO + REGONE (GONER anag)
43D(O)UG
Down
1SCAR + A + B
2AL(B)UM
3CA(R)P
5WICK + I + UPS
6GRE + ET
7OWL (hidden)
8LO(WE)ST
9SANDH + OG (HANDS anag; GO rev)
11S(H)ELL
13BEL PAESE (PALE BEES anag)
15ABAth + SING
18ABE + TTORS (TORTS anag)
20POT + HERB (TO rev)
25SfLATTERN
26MAS + TIFF
27PU(E)B + LO
28N(O)TES (NEST anag)
30DY(E)ING (DINGY anag)
32I + DING
34ADIEtU
37A + MM + O
39pOWEr