What am I doing wrong?

[LCC]

I'm new to polishing and had a question or two. I'm starting out with a 1/2" piece of aliminum. I'm using a bench grinder 1hp @ 3500rpm with 6" buffing wheels. First step was wet sand with 400 in a crisscross pattern, then to 600 wet. After that it appeared In good shape to go to the wheels. Started with brown and got a pretty good finish but some pits and hairline scratches. I thought the scratches were from the brown so I continued onto a seperate wheel with red. Some scratches came out and the shine got a little better but I continued using the red awhile until the aluminum was almost to hot to hold with bear hands. It seemed to get worse after awhile with milky streaks in it. Did I heat it too much? and I still have some hairline scratches. I'm not sure which direction to go now.

Thanks for any help.

[LCC]

After reading some more on the site here, I went back over it with the brown and then to the white (skipped red) with a soft wheel on my drill which has alot more rpm. The results were better but still have tiny tiny hairline grit marks. Are these the swirl marks I'm hearing about? If so I saw how to remove them somewhere on here. at some angles the part looks like chrome but in other angles when the light hits it right I see these tiny marks. Maybe I'm just being picky but I hope not. I'm hoping I'm just missing a step somewhere.

[Rasper]

You are not sanding the scratches out of the metal before you go to the compound. You have to get all of the previous scratches out with each step. In other words: When you are done sanding with the 400, you should have only 400 grit scratches in the metal, and when done with the 600, you need to have sanded out the 400 grit scratches. You may need to start with 280 grit, or even 150, depending on the scratches that are already in the metal. If you don't sand the scratches out, all you will have is polished scratches.

Sounds like you are getting it too hot. I polish mostly stainless, so I am not sure about that.

Richard

[LCC]

Thanks Rasper, that makes sense and I will try that next.

[XK120DHC]

LCC;
I'll add a bit more to what Rasper said.. He is correct in that the sanding scratches have to be all but removed before buffing can begin.. In addition to starting with a more coarse grit of sand paper, continue on thru 600 to 800, 1000, maybe even 1200 if you want that aluminum to shine like chrome..
And most importantly, as you sand (with any grit), unless you are "washing off" the sanding "dust", you are only grinding it back into the surface (creating more scratches!!).. rub your sandpaper across a painted object, now look at the sandpaper.. you have a streak of paint imbeded in the sandpaper.. How well is that streak going to sand the surface?? Or is that streak going to add "gouges" (scratches) everywhere you rub? Well, we can't afford to use one sheet of sandpaper to make one pass on the metal, but we have to do something, and water is NOT a good choice.. but WD-40 is ;-} Just keep the surface you are sanding wet with WD-40 (it will "wash off" the sanding dust and keep the sandpaper from "loading up") and this will all but eliminate the scratches you are putting on the metal while sanding the others off..
Almost Lastly, when you finally do get to the buffing/polishing stage, the piece to be buffed should be so close to being "perfect" that you should start with the "finest" buffing compound and softest wheel.. if it doesn't seem to be doing the job, then go one step more coarse.. and NEVER use two or more compounds on the same buffing wheel.. One Compound=One Buffing Wheel!! (I keep my buffing wheels in labled Zip-Loc Freezer Baggies to keep from cross contaminating them). If you have to, you can go from a "fine" compound to a more coarse compound on a wheel.. but you can never go back to the "fine" compound.. Trust me on this! Also, use Acetone to "wash" the buffing compound off the part before going to the next grit and when you are finished.
And, Lastly, when you are buffing, start out at a 45 degree angle to the left (or right, I don't care) and buff the entire length of the piece like this.. then come back and buff 45 degrees to the right (or left if you started on the right), do the entire piece, then buff the entire part 90 degrees to the length then the last buff will be parallel to the length of the piece..
The "Art of Buffing" is not so much being able to "shine/polish" a piece of metal like a mirror, buffing is creating an "Illusion" that fools our eyes into believing there are no scratches in the metal.. If you could look at a highly polished piece of aluminum thru a very high powered magnifying glass, it would look like "Bad Leroy Brown" after the knife fight (ya gotta be a Jim Croche fan AND old enough to remember the '70's ;-} ).. Doing all that "left-right" cross buffing only "hides" the scratches so we think it's polished smooth.. Neat, huh?
Hopes this helps you.

[LCC]

Thanks XK120DHC, that was good reading & good tips. When you said:

"finest" buffing compound and softest wheel.."

are you talking about white compound for soft aluminum? Then if I needed to, step back to brown?