The dull, powdery look of an aluminium frame IS corrosion - aluminium oxide. In fact it provides protection to the aluminium underneath, and so while you can achieve a shiny aluminium surface by sanding and polishing, it is best not to remove the oxide.
If aluminium oxide starts to form under the window frame, it will expand and then distort and eventually destroy the frame. Moisture will get trapped behind the frame, turning the oxide into a white goo which will damage the GRP.
Aluminium can be painted, but the usual solution is to have it anodised. There are a number of companies advertising in PBO/YM specialising in re-anodising window frames.