Hello Yu Shin,
The exterior paint cost from most of the rental premises in a commercial mall are covered by the landlord mangement company. It is included in your rent or sometimes it will be an additional charge from the lease agreement. You are not responsible for any exterior problem of the building since you lease the space within the building. Anything starting from your storefront to the end of the unit will be your responsible as part of tenant improvement that the landlord allows you to do. You need to read the lease agreement between the lines, especially the fine prints. When in doubt, consult your real estate attorney to go over the contract before you sign. You can ask me anything about the commercial building or tenant improvement concern building code or permit. Thank you for asking.