Why is magnesium polished before it is ignited?
For ease of ignition?
Magnesium tends to burn fairly fiercely once it is ignited; of course you have to get it lit. Exposing a fresh metal surface should aid ignition.
I am told that the engines of old VW engines in the beetles had magnesium casings. Of course this is great in terms of mass, but distastrous results ensued whenever someone tried to weld something onto the casing.