I've had some success in stabilizing zinc based alloy castings (usually carb
bodies) with hard set Araldite. It's pretty empirical, but its always worked
for me so far.
Clean the article as thoroughly as you can - Acetone or something similar
(usual warnings and disclaimers, I suppose, although I expect to be talking
to witful people in this forum!) Set aside to dry in a warm place like the
airing cupboard etc.
Make up a batch of HARD SET Araldite - this is the stuff that takes hours to
dry, not the sloppy junk that you find by the till in B&Q. Warm the article
by placing under a spotlamp. I find this works really well and it warms
through slowly and thoroughly. When its a bit too hot to handle comfortably,
start applying the Araldite to the external surfaces and you'll find that
the metal absorbs quite a bit, particularly if it's old. Carry on until
you've coated the entire outer surface.
Put it aside to cure in a warm place - the grille at the back of the fridge
is ideal. It is usable in 24 hours, but not fully cured for seven days. It
will cure porosity (I used to fix Amal Mk II Concentric carbs like this when
Triumph Bonnevilles got lumbered with a porous batch in the early 1970's)
stop cracks spreading, fill small cracks and minor fractures too if not load
bearing. That said, I have mended broken carb flanges before now and as long
as you reface the flange on glass with wet 'n dry used wet, it will hold
quite well - just don't expect too much of it.
I've no doubt at all that there are better, tougher, smarter epoxy resins
than Araldite and I mention this well known product purely as it is readily
available and I've used it for many years with complete success.
Ford Silver Fox is a good spray can colour to replicate new zinc castings
without going over the top. And it's petrol proof ...