- Defects in the manufacturing process could cause small black specks to appear. These consist of charred plastic, or 'burn', which occurs when the molten vinyl stops moving in the extruder, in which it is made, and "cooks" for too long, causing some of it to burn.
- Again, related to the manufacturing process, if any foreign material is introduced into the extruder, it could cause black specks in the finished product. Common foreign materials include bits of wood, paper, or dirt.
- Cannon, or artillery fungus grows in old mulch and expels spores up to 15 feet high and such rates of speed that the spore actually embeds itself into the finish of the siding.
- As with cannon fungus, dirt can accumulate over time on the outside of vinyl siding and get caught in the 'grain' of the embossing pattern. This can be seen as spots or even larger dirty areas.
- Preventing black spots on vinyl siding can be relatively easy, but not always. Mold can be checked by keeping the area clear of moist mulch or changing your mulch regularly. Once per month cleanings can help keep the dirt from accumulating, as well as just keeping an eye out for new spots and dealing with them as they come up. As far as manufacturing defects go, contact the supplier of the siding for warranty information.
