I believe I can help
I have repeated much of this on many forums so might appear a broken record however here it is for what its worth.
First, DO NOT SANDBLAST AND DONT GLASS BEAD * (*More on GB below)
Castings have a skin to them (Epidermis?) And depending on sand cast, Die cast and quality, the finish may be rough, but its got a hard skin to it. Once you sandblast you remove that top layer and its now semi porous,
## Porosity is variable, Im chiefly referencing the outer side of the cases, meaning every time you touch the cases with oily/dirty hands it leaves a stain, Its hard to keep clean and it will be more prone to corrode easily, (Whitish powder). However SOME castings regardless ARE Truly porous and will weep oil but thats a separate issue and the solutions.
You CAN polish the cases and that seems to skin it back over, and myself, I *LOVE* to polish my alloy.
(Some in the UK are challenged when pronouncing Aluminum, My Yorkshire neighbor runs a machine shop and his staff put it on his "To-Do" list of learn to say it properly)
I have a 65 Triumph with excessively polished cases, head and covers, You need sunglasses.
Prior to widespread use of Soda blasters and vapor blasting, the old school method was called "Bright beading" (or at least thats what everyone I know called it. I bet in Australia they have their own name)
Bright beading is a 2 step process and time & labor intensive. You practice with a glass bead machine with BRAND NEW glass beads, the good quality ones. Take a scrap piece of Alloy and crank down the PSI of the machine, Start at 20-25 PSI and build upl Typically 40-45 PSI but is variable on the cabinet and gun. What you want is just AT the point the glass bead shatters, then drop it a notch so less than 10-15% of the beads shatter. It can take a long time to clean cases... Then follow with 2 steps in a solvent tank, Diesel & Stoddard Solvent makes a good mix although toxic, (Use LOTS of ventilation & respirator) We use 3M Scotchbrite pads, 2 colors, Green & Red, so translate whatever grit that is. The diesel has a lot of oil in it and acts not only a solvent but a bit of lube.
Cases with lots of elbow grease come out sparkly and show ready.
Much easier with Soda blasting or Vapor blasting is my #1 choice. Some positive bennies and comparison.
The soda blasting became the std in aviation as, when done a hot water bath and air nozzle does a good job of removing friction material as it dissolves. (ANY Cleaning you can never be TOO clean, repeat 3x more than you think you need on cleaning) But the soda is the safest method. Soda can be purchased from industrial suppliers and can vary in abrasive # so shop around, The Grocery store stuff works but is too powdery. With soda blast you can blast inside and outside a engine case, all others have to be masked off.
I have been experimenting and found a Harbor freight regular pressure sandblaster works better than their dedicated soda blaster, But you have to convert the gun to a soda model or modify the sand type gun. (Trigger clogs and the nozzle orifice needs to be smaller) I run 120 PSI with a 2 stage compressor with 2 60 gallon tanks and a 90 gallon piggy back tank. Volume matters, (Pressure vs Volume)
I dont care what anyone says, use a respirator, not just a dust mask. I can write a book on the health issues. Sand, baking soda, walnut shells, glass bead,,, ALL of them.
I dont have a vapor blaster but I would like one, Was going to partner up with a friend and build a commercial one, but not on the front burner. We have several places that do the vapor blasting and some guys do ship their cases.
I Prefer vapor blasting if i do case repairs, welding or had to go grinding or other work. A lot of my stuff seems to have holes, nicks and scratches so its rare I dont have to do surgery on a engine case or parts.
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Ye olde junque pileMy old job and my wifes machine shop DO have a texturizing set up. (Blanking on the tech term) But those can vary but they have 2 one runs solvent and one runs water. But its a big hopper about 4' across and cone shaped with a rotator, a spray wand that rotates and spray nozzles on the sides. It has a humongous vibrator table under it, and filled with sandstones and some sort of rubbery cones,,They have different grades of the stuff, just depends on what materials they run thru as the shop runs copper-brass, steel & Stainless as well as alloys so they have different media.
You turn the machine on the bed of stones and other bits shake and you drop your parts in and watch it get swarmed like ants. Stainless takes a while but alloy bits need only 3-4 passes and done. If you do an engine case ALL holes and internals have to be securely sealed off. If you left alloy in there 2 long (overstayed the coffee break) Every sharp edge and feature will be smoothed off. But if you are going to polish the cases,, thats a big time saver! I prep and then have Mrs Sluggo do all my bolts and hardware before I send to plating. Deburs and shines up the metal so it plates beautiful. (its ALL in the prep!)
I dont do it for others except REALLY good friends who gift me lots of beer
But I used to do a lot of polishing and have a backlog of stuff to do. I have multiple buffers and polishers, "Buff-Zilla" is my main machine. Runs a 1" solid billet steel hardened shaft and a monster motor. I can lean into stainless and it wont stall. (Dont get hung up on HP,, its ALL in armature size! Bigger is better. Mass, I got a big industrial motor and its a beast) Ive got stepper pulleys to dial in shaft speed for the material. Buffing and polishing is a whole different art. Im only adequate with a lot of swearing and cursing and occasionally launching a part across the room, But I used to work with and still know some real artists with a buffer. Its truly an art form between a hack like me and a really skilled guy. Nobody lasts long in the polishing business. Carpal tunnel wrist and arthritis is not an *IF but when....
"As cast" finishes are nice, and for a concours show bike, its a standard to uphold, But I love polished old vintage alloy and tend to over do it. Its a special level of OCD because once you start polishing a cyl head or cases,, its either all or none. Half polished looks like S**t.
