Aluminum is a funny material. It's not actually a metal, but it's closer to a metal than a non-metal (technically, it's a metalloid). Regardless, any kind of coating will change its appearance. Powdercoat is nothing more than fancy paint, and it looks like paint. Anyone who's ever looked at Detroit Vintage Wheels' website can tell you their wheels all just look
wrong clearcoated, which is why my Minilite reproductions are raw. There's no real way to preserve raw aluminum's particular look of which I'm aware, and the guys that made my wheels told me they've found nothing themselves (they also refuse to powdercoat their wheels, even at customer request). Any kind of polish will obviously ruin either the rough-cast centers or the spun rims.
As far as cleaning back to the original raw, as-cast look goes, you can get very good results with aluminum hull cleaner and a stiff, non-metal bristle brush. That having been said, you've really gotta want it. First of all, many of those offerings are essentially pure acid and will destroy their surroundings in a hurry. I've heard complaints about the "safe" ones affecting paint, rubber, etc. even though they say "safe for" right on the bottle. The next consideration is the amount of work involved on the manifold itself, particularly on a dual-plane or any manifold with an open area beneath the plenum. You have to be both very thorough and very
fast. Any of those cleaners, acidic or not, will oxidize the aluminum to a dull white in a hurry. The other thing you'll want to consider is the fact that every time you clean it, it's a little more susceptible to staining. You've exposed a fresh layer of aluminum with open pores ready to accept filth or oxidation. Finally, the longer you go between cleanings, the harder each one will be.
Another possible solution would be to anodize it. This can be done
safely, at home if you don't mind doing the setup and getting the needed supplies. Anodizing
will slightly dull the appearance, but it won't yellow nor will it dramatically change the appearance of the alloy. It's a pretty simple process, and as long as you don't shortcut or try "close enough" many have done it with excellent results. That being said, anodizing does not make it super easy to clean. You don't need as aggressive a cleaner, but you'll still have all the labor-intensive toothbrush work. Anodizing closes the open pores on the surface making it harder to stain, but it's still a rough-cast item (no smoother than it was) so dirt will still work its way into all those little nooks and valleys. It just won't soak into the aluminum. Before attempting anodization, though, you'd do well to try a small scrap piece from a junk intake of similar manufacture. Simply put, manifolds are non-wear items so it's not like they're made from pure 6061-T6. The specific alloy of the intake might not take well to being anodized. You would also need to be particularly thorough in the cleanup phase after anodizing to ensure you didn't end up with a bunch of white pockets (excess oxidation) in the low areas of the cast surface. If this is something you're considering, follow the link I included and read the article, a couple times if necessary. Also read through the long Q&A session afterward to avoid easily-made errors. Obviously, in your case you can skip the dye process since you're after a raw look.
The only
simple solutions are to fully polish it (which looks even worse than a raw casting in my humblest of opinions, but it
is easy to clean) or paint it engine color, which you don't like. Maybe I'm misreading what you mean, but I'm not sure why you hate painted aluminum. It looks
exactly like painted cast iron. :dance:
