[leegreen]

oxidation is a natural process for aluminum and once formed it is much harder than aluminum itself, melts at a higher temperature and generally protects the material underneath. Some alloys are more susceptible than others. Your rad might be a better material in this sense or it might have been anodized or otherwise coated with something.

you could try washing it with 50/50 vinegar and water to see if it stops the more objectionable white stuff while a more uniform dull oxide layer forms.

Or paint it black

I've taken to clear coating some new aluminum components like water pumps or alternators before installing so they stay new looking. Clear coating a radiator might cost you some efficiency, adding insulation to the fins? Black color would offset that, many OEM rads are painted black.