Why can't I "trim" weldment profiles with regular extrudes?

It says invalid trim or does not allow me to select the extrude at all.

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2 Answers

Try trimming/extension with a face or faces of an extrude
You can also Trim/Extend to a surface

 
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Also make sure that all the "bodies to be trimmed" cross the plane of the "trimming boundary", otherwise you will get the error

 
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