Originally Posted by DougZ
What do your stretch marks look like? I looked on mine (which has not been buffed) and couldn't find anything. This aluminum thing is getting complicated.
The "stretch marks" I mentioned are kinda hard to describe. They don't show up till you start cutting through the oxide. After a couple passes with coarse polish (you will probably find it will take several passes to get through to clean aluminum--at least I did) you will notice thin dark streaks where the oxide is thicker. I suspect this is where the pure aluminum layer stretched out a little thinner when the panel was formed. It's not a problem aside from the fact that those panels will take more polishing to get the oxide off than the other panels. Or you can do like I did and let some of the oxide stay--I didn't want to polish right through the cladding.
And yeah, you're right--"this aluminum thing" is a lot
more complicated than most people realize! I found a lot of variation in polishing behavior between one panel and another. It's more of an art than a science.
Dale (Pee Wee) Schwamborn worked for Airstream, and purchased their aluminum for a number of years. He has a lot of good stories to tell about it. I've forgotten the details, but I recall Airstream bought the aluminum they used for single-curved panels from one manufacturer (Reynolds, I think) but the aluminum for the compound-curved end shells from another (Kaiser) because the different brands worked better in those uses. (I may have Reynolds and Kaiser reversed.) Nobody knew why.
The other Pee Wee story is that if you look at the pictures of new 60's Airstreams, they looked like they had been polished. They weren't--that's what the aluminum looked like when it came from the rolling mill.
And therein lies a tale. The quality of the surface finish of the sheet depends on the surface finish of the rollers in the rolling mill. The rolls are reground at frequent intervals and the first batches of aluminum sheet that come off the new rolls are better than later batches.
Aluminum is purchased to a specification, and all the aluminum meets the specification. But Boeing paid extra to get the first batches off new rolls so they had the best surface finish.
But there was one relatively small customer that got their aluminum even before Boeing--Airstream.