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resin gives me fits, and don't start about super glue....
Bill, I've been using 'Milliput' for over 40 years, and have yet to find anything to beat it. Available in various colours, but I find the 'Standard' yellow-grey to be the most suitable.
OK, it's a 2 - part epoxy putty, which has to be blended in equal portions, but the filling and moulding properties are excellent, and it can be smoothed with a wet finger tip , brush, or the back of a scalpel bade, before sanding, giving a neat finish. It can also be moulded, tapped, drilled etc, without cracking or flaking.
Stretched sprue melted with Tamiya Extra Thin is good for gaps.