soft varnish
It is a base that remains soft even after being applied on the iron plate. Almost anything that is pressed onto the iron plate will remove the varnish, exposing the iron plate to the acid coating.
en-GB/ˈsɒft ˈvɑːnɪʃ/
en-US/ˈsɔft ˈvɑɹnɪʃ/