The ASTM D3456-18 standard practice provides a method for testing the susceptibility of coatings to microbial attack by external exposure. This practice includes coating preparation, substrate application, and placement of coatings in an external test environment to assess the extent of microbial attack in the actual environment. The standard is intended to guide the selection of external coatings that are resistant to fungi and algae. Although academic studies of specific biological species may be of interest, the actual degree of microbial discoloration is the main concern. This method helps to evaluate the performance of coatings under real environment conditions.