Can you put a plaster on a burn?

Depending on the size and degree of a burn, and the time you expect to wait before reaching a hospital, you should indeed cover it with loose fitting, sterile gauze or specific "burn dressings" that are designed to cover the burn but not adhere. In the event you can't find sterile guaze, protecting an open burn with clean white cloth will work too. The goal is to prevent damage and contamination, without adhering to the burned area. In cases where the size and degree of the burn, coupled with a long transit time cause you to become concerned about patient dehydration, you can consider covering 3rd degree burns or very large, open 2nd degree burns with Cling/Saran Wrap/Plastic Wrap, but I would avoid this unless dehydration was really a critical concern. In the field for a short time, do not cover burns with ointments -- they're hard to remove and tend to pick up dirt. Once undergoing post-first-aid treatment, the attending doctor/medic may opt for ointments such as Silvadine, but this isn't typically a field response unless you're not going to see primary care soon.