Untrained actors make finish for wall (5,4)

I believe the answer is:

rough cast

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'finish for wall' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'untrained actors' is the wordplay.
'untrained' becomes 'rough' (I can't justify this - if you can you should believe this answer much more).
'actors' becomes 'cast' (group of actors).
'rough'+'cast'='ROUGH CAST'

'make' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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