A method that works with green stuff is to slather vaseline on the part being fitted in (wing, arm, shield, etc...), fill the gap with green stuff and sculpt to match the surrounding area, let dry.  Once dry, you should be able to separate the pieces as the vaseline will have prevented the green stuff from sticking. They key is to let the green stuff cure before pulling the parts apart. Once that's done, wash the parts thoroughly with a grease cutting dish soap.  Prime and paint at your leisure.
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