The performance of a heatsink with a black surface

[digsys]

Anodizing and Alodining have excellent heat transfer properties. Powder coating is a bit trickier. What you lose in heat transfer, you gain
by not having Al oxide as an insulator. In general, black powder coating is nett beneficial. The only "bad?" characteristic (in some cases) is that
there is a slight reduction in rate of thermal transfer, but the FINAL effectiveness is about the same. IF you need a fast transfer rate, just
upgrade your heatsink size / type a notch.

[edavid]

Dragging up an old topic here but..
Say I have a Heatsink which is painted or powdercoated on all sides, and I want to mount my TO220 reg onto the heatsink. Sure I'm gonna use thermal paste + mica pad and a nut & bolt to fasten it but is it necessary/beneficial to scrape the paint from the contact surface of the heatsink to transfer the heat better?

You should never paint or powder coat a heatsink!