aluminum to a bright almost mirror finish really isn't that difficult.
It just requires some elbow grease and plenty of time. When working with aluminum, keep in mind that it is a
relatively soft metal that scratches easily so when smoothing or
polishing, be careful which way any swirl marks may appear. The plates
I polished were certified 6061-T6 1/4" aluminum sheet. I spent about 3
hours or so on the mid-plate and another hour and a half on the front
side only of the front engine mounts.
If you were to look at the raw aluminum under a microscope,
you would see what could easily be represented as a saw blade with
ridges, peaks and valleys across the surface of the metal. The process
of polishing basically reduces the height of the peaks drastically
reducing the depth of the valleys. This allows the valleys to be cleaner
and reflect light rather than absorb it.
I started the process by dry sanding the plate with 320
grit automotive sanding disks (available at any NAPA store) using an
orbital sander. I sanded each side as smooth as possible using 3 or 4
disks per side. After this first step the difference in smoothness is
very noticeable. The next step was to wash the plate thoroughly with
Simple Green and water. Between each step it's very important to remove
all of the black cuttings and keep the plate clean.
The next step was to wet sand with 1000 grit hand sheets
using mineral spirits as a lubricant (not water). Mineral spirits
suspend the cuttings allowing them to be washed away so that the don't
clog up the microscopic valleys. Keep your motions going in one
direction for a more even finish. I used 3 or 4 sheets per side. After
completing this step, wash the plate thoroughly with mineral spirits.
Repeat this entire step now using 1500 grit then repeat the whole step
again with 2000 grit. For an even brighter shine, you can do it one more
time with 2500 grit, I didn't. I buy Carborundum sanding paper from McMaster-Carr