What grit can I use?

All the grits you want BUT

Stick in the beginning to 0.2-0.5mm (there is a conversion table on the website)

Do NOT use cheap white sand that you buy for your children to play in. That sand, when it breaks or is blasted, releases silicata which is EXTREMELY dangerous

We have good experience with Metabo grit Aluminium Oxide, available on Amazon and others