They may have been pickled and zinc chromate primer prior to painting, hence the greenish tinge that Andersonix noted. This would mean they are not anodized.
Anyway, best not to try and etch off any anodizing if present, it makes for a paintable surface once cleaned unlike regular aluminum. Use an oil paint once the old paint is off if in fact it is anodized. To check if it is, just do a continuity test on the surface. Anodizing doesn't conduct. To double check, dig the probes in beyond the depth of the anodizing to confirm conductivity.
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I finally got all the paint off the heat sinks with more Aircraft Remover and some brushes from Harbor Freight. A lot of work. Turned out to be a great sounding Firstwatt F5.
