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If you cut your own parts from aluminum, be sure that you've removed every
tool mark on the edges before you do any finishing, such as alodine or
anodize. It's also a good idea to slightly break the edges a couple of
thousandths too. This is important to remove any stress risers that might
occur at a tool scratch or gouge. A good method for this is to draw-file the
edges with a set of good quality fine tooth files (get a set of Nicholson SS
Pattern Files) and finish off with green Scotch-brite until bright and
smooth.

Scotch-brite, with soap and water, is also excellent for cleaning the part
surfaces (aluminum) of oils and grease, such as finger prints and cutting
oil, prior to acid-etch surface preparation in Alumaprep 33 or equivalent
(for alodine). Also, unlike any kind of sandpaper or crocus cloth, you'd
have to work very hard to cut through the AlClad (if you're using that kind
of aluminum stock) with Scotch-brite. If you get your parts anodized at a
quality shop, they should do a thorough prep job before the anodize
treatment but to be sure, it can't hurt to clean the parts yourself.

There are at least two basic types of anodize treatment (I don't know what
the difference is as I'm not a process specialist) but from experience, I
prefer to request a "hard anodize" since it it imparts a nearly
un-scratchable surface. It also is an excellent insulator which should
impede dissimilar-metal electrolysis where the Al part comes into contact
with other metals. If you don't specify, your parts will probably come back
medium to dark gray but I have had hard-anodized parts colored black and I
think other colors are available.

If you have parts with interference-fit holes anodized, like pre-drilled
rivet holes, be aware that the process builds up a bit on the surface and
holes that were perfectly sized before the treatment won't be afterwards.
They'll have to be a thou or two oversized (check with the shop on the
build-up expected) before the anodize.

Unlike alodining, which is a paint prep, painting an anodized part is just
gilding the lily. A good anodize surface is nearly impervious to the
elements, and if you can get it in the color you want, looks better than
paint too.

One final thing, the aluminum alloy used to make your parts will (or can)
affect it's final appearance and maybe it's acceptance of the anodize,
though I believe the types commonly used in our aircraft look and react just
fine. Again, ask the shop, they ought to know.

Dan Schaefer

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Please send your photos and drawings to marvkaye@olsusa.com.
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