Paint formulations contain standard industrial biocides for in-can preservation and stability. However, in-can preservatives do not necessarily offer dry film protection, as the dynamics of microbial protection change when the paint film dries. Furthermore, Environmental and Protection Agencies registrations and regulations dictate the kind of antimicrobials that can be employed for dry film protection and claims that can be made from the use of these antimicrobials.

Formulating paints that have long-term dry film protection is challenging. For outdoor applications, variations such as the direction a painted surface faces can create dramatically different responses to how the paint is attacked by algae and fungus.

There are a number of laboratory-based test standards available for the determination of antifungal and antibacterial performance of dry paint films. These allow evaluation against standardized microbial challenges under predictable and reproducible conditions. A number of these methods may be specified by designers, architects or standards organizations, but it is important to understand what aspects of antimicrobial performance they are intended to evaluate. For example, in an antifungal test method such as ASTM G21, samples are placed horizontally in contact with a nutrient-poor salt agar and directly inoculated with a selection of fungal spores.

Another ASTM standard, D3273, exposes samples to an airborne selection of fungal spores by vertically suspending them above an inoculated soil bed contained in a warm, humid chamber. These tests can express the antifungal attributes of dried coatings and paint films in drastically different ways, and coating formulators must, therefore, take care in selecting test standards that are most relevant to their end-use application.

For outdoor paints, there are ASTM standards that prescribe realistic outdoor exposures for expected use conditions.

Factors that can impact performance such as surface angle to the horizon, the degree of surface condensation, the degrees of shade, acid rain and pollution, are taken into account by these methods.