I painted an outbuilding last year and it has mildew on it already.Is this guaranteed to not mildew??

Thank you for your question. Our products contains mildewcides to assist with resisting the growth of mildew. This will not stop any growth that was already existing within the spores and will not stop all growth. We always recommend cleaning the surface thoroughly with detergent and bleach prior to application and continue with annual cleaning to ensure contaminants do not become a concern.