How to prep for polishing?

[dbw360]

I've got some aluminum I'm going to polish, some with nothing done after casting, some pretty beat up and needing sanding, and some with a machined finish. What preparation do I need to do before starting with a sisal wheel and emery compound?

[mcaswell]

Besides that, there's no prep necessary.

[dbw360]

How fine should I go with the sandpaper? Also, how do I know when to stop with the black compound/sisal wheel and to move on to a sewn cotton wheel/brown compound?

[mcaswell]

Use sandpaper until the part is smooth to the touch.

Use the black/sisal until the part looks pretty much perfect, then move on to the brown/cotton.

You'll be doing 95% of your buffing with the black/sisal combination.

[dbw360]

OK, thanks. I think I'm not spending enough time with the sisal wheel. One more question, one of the things I'm polishing is the tops of some pistons. I want a pretty smooth and shiny finish, but don't want to removed much material. Any idea on how much buffing with the sisal wheel and black compound can remove?

[mcaswell]

That really depends on too many factors to make even a close guess. Factors include:
<ul>
<li>The amount of compound used</li>
<li>The type of metal</li>
<li>The pressure you apply</li>
<li>The speed of your buffer</li>
<li>The size of your wheel</li>
</ul>

[dbw360]

OK, I realize you can't give an exact measurement without a lot more info. How about asking it this way: Given a smooth to the touch surface, but not "polished", how much metal typically is removed to get a high polish (aluminum in this case). Or maybe another way to think about it, what grit of sanding scratches are easily removed by the sisal wheel/black compoud in aluminum (320, 600, 1200, ...)?

[mcaswell]

You're trying to be very specific here, and I appreciate that , but you really can't be.

Polishing isn't a science, it's a skill, much like painting. You polish by eye and be feel, as opposed to by measurement.

Using the sisal and the black compound you can remove as much metal as you want. The more you polish, the more is removed. So the answer to your question, technically, is that any size scratches can be removed.

The trick, however, is that depending on the depth of the scratches, you might be better using a finer grade of sandpaper first, to save time with the black compound.

Obviously, if you work your way down from 320 to 1200 grit, you'll be saving yourself a lot of time with the black. However, if you just sand with 320, the black will still work (and will remove lots of metal) but it will take you much longer.

If you don't want to remove too much metal, and need to be sure about how much metal has been removed, there are many low tech and high tech measuring devices that will tell you how the thickness of the metal. This is really the only way to tell how much is removed....you can't tie it down to any average numbers because of all the factors previously mentioned.

That said, you aren't going to see big chunks of metal flying off the part when using the sisal and black combo....it's a gradual process.

If you want it to be an even more gradual process, step down to the brown compound, which contains less abrasive.

A question for you would be - how much metal is acceptable to lose?

[dbw360]

caswell wrote:
A question for you would be - how much metal is acceptable to lose?

Probably no more that 0.010".

[mcaswell]

Then you need to be very careful. I'd skip the black and go straight to the brown, and buff in very quick, short segments.