Best illustration is crustyness here. The metal frame around the facia. The chassis is as you mention die cast aluminium.
Ah, gotcha. That style of front surround is an aluminium die casting. They look like they are anodized but are in fact painted with an aluminium paint that gives them a very similar appearance. Unfortunately the paint (albeit some of the best ally paint that I've encountered) is nowhere near even beginning to become as hard or tough as anodizing. The typical crustiness on these is where the paint has worn and is exposing the underlying die casting. Sometimes it's where little smears of some other metal have been deposited onto the paint by contact with other bits of kit. The paint is good, very good, and it's almost impossible to distinguish from anodizing until you've seen somewhere it has chipped off or worn through to the base metal.
Aluminium die castings themselves typically anodize very badly. The alloying elements used to make them good casting alloys tend to 'smut' the surface of the aluminium and show through in the anodizing, way beyond the power of de-smutting treatments to remove. I'm guessing that's why the otherwise odd combination of ally with ally paint was used - they wanted the appearance of an anodized finish (perhaps to match other components) but couldn't manage to get both a good casting and good anodizing.
Just to confound matters,
unsealed anodizing is sometimes used to make a good surface key for painting. In this case the cosmetics of the anodizing don't matter (its going to be covered in paint) and the thickness of the anodizing is much less. I further suspect that is exactly what HP have done here. So you've got a die casting, non-cosmetic anodizing, and then aluminium paint specifically picked to look like anodizing (theorizing - based on what I've seen).
Just to compound the matter, I'm not sure if the above is
always true of that style of surround. I've seen some that I would swear are actually anodized. I don't know if this is because they got the finish of the painting method down so good that I'm fooled or whether at some point they switched to a die-casting alloy that could be satisfactorily anodized. And I've seen some that look like a bare, but good, casting that has just been grit-blasted in some fashion.
All of the above makes renovating the surface a great big pain in the arse. If you use anything abrasive to remove contamination it might strip off bits of the paint, go too heavy and it might cut through the relatively light 'adhesion' anodizing and expose bare shiny metal. Trying to re-paint/over-paint it? Na, the original really is so good that you won't even begin to reproduce it without access to a proper industrial paint shop.
My best guess is that you would get the best result and do the least harm by the judicious use of scotch-brite. You need to be careful here, as not all scotch-brite is the same. It come in many grades, and there are many knock offs (and some middling to good brands like Mirka) that you might get sold as scotch-brite and then what grade you've actually got in your hand is anybody's guess. I would say that for this pukka Scotch-brite 7447 (a dirty maroon coloured, officially 'very fine') is the coarsest that I'd risk - I just went and tried a piece on an inconspicuous corner of my 3325A which has the same kind of surround and it didn't do any harm. If you want to PM me your address I'll stick a sheet in the post - I have lots. The 7448 grade ('ultra fine' grey base colour) might be a better option but it's harder to find in less than full-sized boxes (20 sheets). Which coincidently I'm waiting on backorder for, you can have some of that too if you can wait for a few days.
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Topic: Test Equipment Anonymous (TEA) group therapy thread (Read 18625122 times)