Aircraft grade aluminium is an alloy once known as duralumin that contains copper and manganese. It is stronger but corrodes faster than 'pure' aluminium.
To combat this, the raw sheets are coated with high grade aluminium. Scratching and polishing are best avoided as much as possible (American Airlines liked to polish their aircraft). When I was an Apprentice with BOAC we had to carry out various repairs using duralumin without damaging the protective layer. If you scratched it, you had to start again. Sometimes, days of work went in the bin.
Paint will be far more durable.
**Aluminum/Aluminium