There are so many different ways to finish metal, it would be difficult to go over them all in a single lesson. We are going to briefly touch upon some of the most common methods, and then try out one of them ourselves. Some processes such as bluing, electroplating and powder coating are typically too difficult to do at home, and are usually outsourced to a production house.
Like most other solid materials, metal can be painted. Typically an aerosolized (spray) paint is best and applies the most event coat of paint. With paint you can achieve a glossy or matte finish, as well as a bevy of other painting effects.
A nice thick solid coat of paint is also good for metals which oxidize, as it seals them and prevents oxidation. It also protects against light surface scratches, as it is much easier to touch up paint than the surface of the metal itself.
You can see metal painting in action in the Napparatus instructable.
Sanding metal (particularly non-oxidizing metal such as aluminum) is an easy way to bring it to nice dull luster. This process starts with a rough grit sandpaper around 220, and progresses incrementally to a fine grit sandpaper around 2000. At each step, finer and finer gouges and surface imperfections are removed until it gradually achieves a nice smooth luster. Oxidizing metals such as steel would still need to be treated with a clear coat of sealant after sanding to prevent rusting.
If you really want your metal to shine, you need to polish it with a buffing wheel. Polishing metal is typically just a continuation of the sanding process. Buffing uses compounds, which are pastes applied to the buffing wheel. Each compound is a finer grit. As you progress through the process, the surface of the metal will get smoother and shinier. Using the proper buffing compounds, buffing wheels and metal polishes, you could bring many types of metal to a mirror finish.
Sand and Polish
We are going to finish our kaleidoscope tube by sanding and polishing it to a nice reflective shine. This process is messy and a little bit time consuming, but adds a level of professionalism to your work that is ultimately very rewarding. By the time you are done, you will be surprised how much this process can transform a piece of standard aluminum stock metal.
Before you do anything, put on rubber gloves to keep your hands clean, a dust mask to keep your nose and lungs clean, and a work apron to keep your clothes clean. You will thank me later.
Begin sanding your tube with 220 (or equivalent) grit sandpaper. You should begin to see the layout fluid, identification markings, and other imperfections begin to disappear. Continue sanding until you get a nice uniform surface on the tube.
Also, sand the face of the cut edge of the tube. Feel free to sand the edges to round them out a little, too.
Do not sand the face of the side of the tube you have not cut, nor round these edges. We are going to be gluing the eyepiece flush to this side, so we want to keep it square.
Repeat this process with progressively finer grit sandpapers from 220 grit up until 2000 grit, spending a few minutes with each consecutive grit. A typical progression might look something like 220, 320, 400, 600, 800, 1000, 1500 and finally 2000.
Once completed, the tube should now have a nice smooth dull luster.
Insert the polishing wheel's shank into your drill bit chuck and spin it up to full speed.
Rub the spinning wheel along the green buffing compound until you have a nice coat upon the wheel.
Clamp the aluminum into the vise, and while spinning at full speed, press the wheel down upon the surface of the tube.
Move it back and forth to polish it.
Resume doing this until the whole tube has been buffed and it has a nice shine.
Quickly wipe the tube down with your shop rag when done to remove leftover buffing compound. You don't need to do too great of a job right now because we are about to buff it again.
While the green buffing compound made things nice and shiny, it's really meant to remove shallow surface scratches and imperfections.
To really make it shine, you want to use the pink scratchless buffing compound.
This is applies to the tube in the same manner as the green buffing compound. Continue doing this until the outer body of the tube is polished.
Wipe it off when done. It should be fairly shiny.
If you want to get it really - really - shiny, it is better to use a real buffing wheel that spins with more torque at higher speeds. This shop tool really is able to polish the metal much more and really smooth it out. It also creates much more heat, which generally helps this process. That said, the drill mounted wheel will do for the purposes of learning.
Clean It Off
Even after wiping it with a rag, there is going to be plenty of residual buffing compound left on the surface. In the old days I would remove this with a rag coated in acetone. However, it's better not to work with acetone if you can help it, and this is a neat trick that I learned which is just as effective.
Fill a bucket or plastic container with hot water and pour in a little Simple Green.
Drop your part into the water and walk away for a few minutes.
Dry It Off
When you come back, most of the gunk that was on the part will now be floating on the surface of the water.
Remove the part from the water and wipe it down with a microfiber cloth to dry it off and clean off any residual gunk.
Your tube will now be oh-so-shiny.
Congratulations on finishing! It is now ready to be turned into a teleidoscope.