Is it fine to cover the wound with a medical plaster? because every time i do, the wound tends to moist and it takes longer time to heal.

Need more info: Wound care has advanced a lot in the past few years. I need more info to be able to answer your question. Do you go to a wound care center? If not, consider it.

Moist is better: For superficial wounds (burns/scrapes that don't go all the way through the skin) keeping the wound moist results in faster healing. Don't confuse "time until a scab forms" with "time until the wound is healed!" Yes, once a scab forms you don't need to dress it, but that is not the same as healed. Healing is not complete until the scab comes off and there is skin across the wounded area again.