Charles,

Here are my "two cents", as a sculptor creating outdoor pieces in
aluminum:

Since you want the appearance of bronze, but you are planning to work in
aluminum, I assume that the sculpture will be large (and the choice of
aluminum is a budget choice).  When I have looked into anodizing, it has
only seemed cost-effective on small objects.  If you are talking about
working with pre-anodized sheet material, that may be a different story,
but then, presumably, you won't be welding the edges, since that would
spoil the anodized surface.

I haven't tried to simulate bronze on aluminum with a patina/coating,
but my general concern about patinas, paints, and other coatings is that
sooner or later, they are likely to become faded, chipped, scuffed, or
de-laminated in some way.  If a repair is required, matching the
original finish can be difficult.  I favor "brushed" aluminum surfaces,
which can be re-touched relatively easily.  One option for expanding
beyond the natural color of aluminum is to incorporate other materials
(bronze, colored glass, etc.) into the design to create contrasting
colors and textures in detail areas.  Perhaps you could use aluminum
with bronze details(?).  Perhaps a smaller sculpture made in bronze
would work(?).  Of course the choices depend on the design and the scale
required by the site.

If you find a long-lasting bronze-like finish for aluminum, I'll be
eager to learn the secret.  ;)

Harry McDaniel
HarryMcDaniel.com