How to reduce the partsize by a layer of 0,5mm?

 From:  simba
6921.1 
Hello,

I need to reduce the size of a part (blue wing section in the attached file) by a layer with a thickness of 0,4mm to provide room for a 0,4mm coating applied after machining the part.

I have been playing with negative values in offset without success.

How can I achieve this?

S.
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