Buy some silver paint, and a fine paint brush.
Dip the brush in the paint.
Wipe the brush on a piece of paper until all the paint is off.
The very lightly, brush over scratch.
Repeat until happy.
For more info, google dry brushing.
*I once did this to a macbook alu I listed on ebay as immaculate and the buyer didn't notice
Do you have any before and after pictures by chance?
I tried googling around, but I couldn't find anything that suggested this was a method of filling scratches. All I found were descriptions of how different drybrushing is aesthetically and discussions of famous painters who used this technique in their artwork. Do you have a guide or favorite website that explains it?