"mottled effect" on anodized front panel


Hi everyone, I recently made a really great purchase of a Tektronix 536 together with a 132 (single slot plug-in power supply) and 5 plug-ins with 2 sets of all the manuals. Amazing most items are in extremely good condition, and 3 of the plug-ins even come with their original cardboard holders (which I presume were used when new - look old!) and even better the original sales receipt for when all the items were resold back in the 80's to the person I purchased them from. For me provenance is almost as interesting as the equipment itself, so this was a rare and good find!

In any case, coming to the point of this post - despite the good condition of all of the items, on closer inspection the scope has a slightly mottled effect on the anodized front panel. It does not show up well in a photo, but from most viewing angles the front panel seems to have lighter and darker areas - all the more noticeable on the 536 as large areas of the front panel are not used. Hence my description of a "mottled effect". The instrument has no corrosion or other indications of improper storage. I've tried the usual soapy water but it makes no difference. Anyone know what this may be, or have any ideas on how to remove it?

Thanks for your help.

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