What the heck?

[mpierich]

This sidecover was leveled with 120, then I used 220/320/400/500 & 600, all on a plam sander, Everything after 220 was wet-sanded. Then emery/bias-cut, emery/loose, and white/loose.

Still it has horrible shadows, almost like metalflake under the surface (which is completely mirrored except for this problem).

Any suggestions, ideas, ever seen it?

Some of my research indicates that once you break the surface of a cast item, you're always into micropores - whether they show up only depends on their size and shape.

I can't wait to explain this to my customer...

[sswee]

Not alot of help but I've seen it before on the mc parts I polished and anodized in fast blue. On a couple of them, it had the appearance of a loose metal grain. Hard to explain to someone that doesn't work with metals. I don't think there is much you can do with it or about it.
SS

[jimcarry]

I have also seen this
assuming that this occurred after polishing this would be my suggestion
some metals like aluminum, pot metal will do this when they reach high temps
I had this happen on some Mercedes wheels I polished try sanding out a small area on the wheel
to get the smooth look back then when polishing use more compound and less pressure
try not to get the part as hot let the compound do the work

[fly]

I would do like Jim said but when you get to the final step try the blue/flannel cotton, I had a similar problem with a bike frame and the blue/flannel helped me out....

I have also seen this
assuming that this occurred after polishing this would be my suggestion
some metals like aluminum, pot metal will do this when they reach high temps
I had this happen on some Mercedes wheels I polished try sanding out a small area on the wheel
to get the smooth look back then when polishing use more compound and less pressure
try not to get the part as hot let the compound do the work

[mpierich]

OK, thanks gentlemen, I'll try dropping back to 400 and give it another shot. I'll do it in one area and report back.

[mpierich]

Well, I started back at 320, used the same sanding routine but then loose/emery and loose/white. Used twice the compound and half the pressure and more time. There was no heat buildup, but I got the same result. The more glazed the surface became, the more the haze showed up.

So, it's either a flaw in my sanding, or pores/grain in the metal. I might try stopping at emery if I can get the wheel marks out, but it doesn't look good.

Times like this I feel like selling my equipment...

[fly]

Hang in their it will get better.

[pickleboy]

dont give up mike get in touch with me when you can ok bro.

[mpierich]

dont give up mike get in touch with me when you can ok bro.

OK Cliff, just checking in here quick before I go out to the shop....sorry I ddin't answer your email yet but I will, havin' a tough time right now.

[thesound]

These tiny specks you're seeing in the finish are called inclusions. This text gives a pretty good idea of what they are and how they get there:

Effects of Inclusions. In addition to oxides, a number of additional compounds can be considered inclusions in cast structures. All aluminum contains aluminum carbide (Al4C3) formed during reduction. Borides may also be present. By agglomeration, borides can assume sufficient size to represent a significant factor in the metal structure, with especially adverse effects in machining.

Under all conditions, inclusions whether in film or particle form are damaging to mechanical properties. The gross effect of inclusions is to reduce the effective cross section of metal under load. The more devastating effect on properties is that of stress concentration when inclusions appear at or near the surface of parts or specimens. Fatigue performance is reduced under the latter condition by the notch effect. Ultimate and yield strengths are typically lower, and ductility may be substantially reduced when inclusions are present.

Hard particle inclusions are frequently found in association with film-type oxides. Borides, carbides, oxides, and nonmetallic particles in the melt are scavenged and then concentrated in localized regions within the cast structure.

I discovered this phenomenon while in the midst of polishing a cast iron engine block. The block was cast as a "high nickel" block, which by default designated all the nickel in the parent iron as an inclusion. The consequence of having nickel, which is a pretty soft metal, dispersed evenly throughout the harder iron casting in the form of tiny specks, was that when I got to the buffing stage after sanding to 600 grit, I discovered that the buffing compound was rubbing away the tiny nickel speckles quicker than it's harder iron counterpart. This basically left me with a continuously progressive "orange peel" effect that never smoothed out no matter how much compound I used or for how long.
This of course gave me fits, but it also forced me to be experimental. I exhausted pretty much every form of buffing wheel using different wheels in combination with different compounds, and had a marginally higher degree of success with certain combos but nothing that actually amounted to the problem being solved. Due to the rules of this board, I have no choice but to PM the details about the product I used to the originator of this thread, but what I can say is that this stuff made the difference between the project being a success and a failure, and I'm pretty confident that it will help with the problem here.