Aidrian Bridgeman-Sutton's extensive observations on this subject are
right on, and obviously based on both thoughtful consideration and
extensive experience. I agree wholeheartedly with both his philosophy
and his practical suggestions. Implied, though not said in so many
words, is that finishing models is an art form which cannot be imparted
through simple formulae or "handy hints," and what works for some of us
doesn't work equally well for everyone. Remember the old ads for
vocabulary-building programs which promised "thirty days to a more
powerful vocabulary?"
Well, as a professional linguist I can tell you that "thirty years to a
more powerful vocabulary" is closer to the truth, and much the same is
true of weathering models. There's no substitute for experience, and –
as has been said repeatedly on this list – a good way to start is to
pick up some bargain-price junkers at a swap meet that you can
experiment with and, if the results are a disaster, pitch into the
trash.

Richard Hendrickson