Like your defog capability in your car, the efficacy of mask defog is effected by temperature and surface tension conditions on the surface of the visor. Everyone knows surface tension will cause fogging to occur at different levels. Of course, we don’t want to apply anything to the visor and that’s a feature of the Guardian mask. However, like your automobile windshield, temperature and moisture on the visor prior to the dive will affect the level of clearing that can be expected. In your car you notice that it takes some time for the engine to warm up and therefore produce hot air before defogging occurs. In a diving mask, we don’t have the luxury of blowing warm air over the lens. So, we rely on air blowing over the surface of the visor to evaporate the moisture.
Problems may arise when the inner visor surface is wet before the dive. Water on the surface of the visor, without any anti-fog agent can be problematic. This is because the only way to remove the moisture is to completely dry the surface of the visor. Since we are no longer talking about a mist of moisture but rather much more water it is obvious it will take much longer, if ever, for the visor to become dry enough for defogging to occur. Fogging issues are rarely a problem when the dive is initiated with the inner surface of the visor dry. There can be some exceptions where just the right conditions occur and defogging is more difficult. However reports of this happening are rare indeed. If this condition occurs, then perhaps the remedy is to use a defog solution. If you dive and have persistent fogging, and it’s due to a wet visor inner surface, it is advised to surface and have the visor inner surface dried with a soft non-abrasive towel. I have personally experienced this pool side. The only way to get the mask to defog was to dry the inside of the mask. Worked every time!