Improve one's knowledge of Polish (3,2)

I believe the answer is:

rub up

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This clue might be a double definition.

'improve one's knowledge' is the first definition.
'rub up' can be an answer for 'improve' (thesaurus). I am not certain of the 'one's knowledge' bit.

'polish' is the second definition.
(I've seen this before)

'of' is the link.

Can you help me to learn more?

(Other definitions for rub up that I've seen before include "shine" , "Polish" .)

I've seen this clue in The Globe and Mail.

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