What problems can arise from using glass bead as a blasting media?
I'm no expert in anodizing, but apparently nobody else in America is either, so I decided to do it myself on my own product line. Take the following advice at your own risk, it's just what I have observed during a long and frustrating saga.
Any kind of blasting will embed particles of the blasting material in the soft aluminum surface. As MachEng said, if you feel you have to blast, aluminum oxide is your best bet. This is because part of the prep in the anodizing line is etching; a dip in sodium hydroxide (lye) which removes the outer layer of aluminum oxide that is formed on the surface of all aluminum alloys seconds to minutes after it is cut, ground, sanded, etc.
If the blasting grit that is left behind is aluminum oxide, it has the best chance of also being removed during the etching process, but I've found it to still be hit or miss. The surface can be very splotchy and unpredictable, and the larger the area, (such as your shoebox size part) the more pronounced it can be. Extending the etching time may help this, but it does remove material, so you have to control this as well.
I gave up on blasting, and now only tumble my parts in plastic media with water running through it. The problem with your parts is the size and shape is not going to be very conducive to that, and if the have sharp corners, you're probably going to have to do them one at a time.
Since they are high dollar items, I would probably experiment a few different ways (on scrap of course). If they are flat-sided, I would try wet/dry paper spray stuck to a surface plate and lightly lap (600 grit) to remove milling marks. Then I would etch them myself to a consistent looking finish before sending off to the anodizer. You can control the temp and time in the lye to get what you want visually, plus the added benefit of being able to measure between "dunks" to see if you're going to far. If the lapping is not practical, blasting with aluminum oxide on a low pressure setting would be the next process to try.
If the grayness is coming off with elbow grease, your anodizer's sealant tank is probably getting long in the tooth. What happens is after repeated heating cycles, the desmut chemicals come out of solution, and residue is left on the parts. The good news is that it still seals fine, it just looks like crap and you do have to remove it manually. I just bought new sealer yesterday because of this. I was using food grade mineral oil on mine, but still didn't look quite right. Tell your anodizer to call U.S. Specialty Corporation (704-292-1476) and order a gallon of Specialty Ni Additive for $50.00. This way he won't have to put down his crack pipe and drain his tank, he can just dump this right in (16 oz./100 gallons I believe).
Yesterday I got a wild hair while waiting for the new sealer and used Meguire's heavy cut #4 automobile buffing compound (water based) to remove the smut, and it worked like a champ. The surface is very smooth to the touch, not polished, but with a sheen to it. This may be too much reflection for your application, but it sure does look nice.
If you do pre-etch, try and keep from handling it too much afterwards, use rubber gloves to keep fingerprints/oil off it, just in case the guy's not cleaning it too well. Clean with a non-lye based product (purple degreaser a big no-no) such as Simple Green or Dawn dishwashing liquid (or both in that order). Rinse with de-ionized or distilled or reverse osmosis water and dry before sitting it down somewhere. Setting it down wet even on a non-metal table can caused discoloration where it's been sitting a even a slight bit of water.
Hope this helps, but if this is a product you're going to be producing long-term, the best solution is to either set up your own anodizing line, or find a competent anodizer. Both are going to be a lot of work, but the last might just prove impossible.
Good Luck,
Burt