HSN/Sac code for works contract

Sir
we do job work namely Hot dip galvanizing.
Coating of zinc on steel.
value addition to the principal input from party.
Galvanising amounts to manufacture earlier and we collected Vat on zinc transfer.

Now in GST can we consider our job work as Service..
if yes then Sac code?
if no then what is our HSN code with zinc being our primary purchase.