The most common problem associated with polishing aluminum is the deep scratches that plague the finishes. Three things primarily cause deep scratches in aluminum:
1) The wrong type of abrasive being used.
2) Too much downward pressure from the polisher’s hand.
3) Using too long a stroke while stoning.
Aluminum is primarily stoned by hand and easily turns to powder as it is stoned. This powder constantly clogs the pores of the stone and then builds up to “score” or “tear” the aluminum. Efforts to leave stones and use sandpaper yield somewhat better results. This method is the most commonly used today, but even stoning with sandpaper can leave a long, tough road of deep scratches, which refuse to diamond out properly.
The answer to the first problem is to use newly developed stones that are specifically made for all nonferrous metals (e.g., aluminum, copper and brass). If used with mineral spirits as the lubricant, these stones will never tear the metal! Even using these new stones while purposely trying to cause rips and tears won’t cause a tear. Check with your favorite stone vendor for samples of these newly developed stones for non-ferrous metals.
Like most sanding jobs, you’ll want to start with a coarser grit sandpaper and work your way down to a finer one. With the exception of something like a truck tank, this will require nothing coarser than 320 grit abrasive. However, if you have deep gouges you need to get out, you may need to go to a lower grade. We do not recommend going below 180 grit for your abrasive.
You will work from the lowest grade which would be 180-320 depending on the severity of your project and work upwards until you have the deepest scratches or gouges sanded out. Many professionals recommend going from 320 grit, to 400 grit, and finish with 600. The real detail oriented polishers will sometimes go to 800 or even 1500 grit before moving onto the next step.
Smaller aluminum pieces can definitely be done by hand with sandpaper and a little elbow grease, but when you get into bigger projects, you’ll want to use an abrasive with a power tool. You don’t want to rush this process, but you also don’t want to spend hours and all of your energy sanding by hand. The best option for speeding up the sanding process would be to use an orbital sander with PSA discs or an angle grinder with an aluminum grinding flap disc.