Brick and Mortar
Use chemical-based products such as Jomax or Brick and Mortar Cleaner to remove rust stains and creosote build-up from chimneys. Whenever choosing a chemical-based product, always follow manufacturer's recommendations on mixing, storage, disposal and safety precautions.
A pressure washer is an environmentally-friendly alternative to chemicals. Use a 0- or 15-degree nozzle and a medium-pressure unit to pressure wash the rust stains from the chimney.
Vinyl or Aluminum Siding
Rust stains on masonry chimneys that have been covered with vinyl siding can also be removed. According to the Vinyl Siding Institute, Fantastik, Murphy's Oil Soap or Windex will remove rust stains.
In addition, cleaning siding to remove rust stains will also remove creosote stains from the chimney. A pressure washer may also be effective.
However, use a wide nozzle, such as a 25-degree, on low pressure to prevent putting holes in the siding. Chemical-based products that are used for cleaning brick and mortar may also be used to clean rust stains from chimneys covered with vinyl and aluminum siding at a weaker concentration.
The concentration requirements are printed on the package.
Many paints are developed with the same types of chemicals that are used in vinyl siding. Use the same cleaning products suggested for siding: Fantastik, Murphy's Oil Soap or Windex.
Pressure washing may also be effective. However, use a wide nozzle, such as a 25-degree, on low pressure to prevent peeling the paint off the surface.
Make sure your ladder is sturdy, level and on solid ground. Wear eye protection to prevent chemicals from cleaning products or spray from a pressure washer from getting into eyes.
If using strong chemicals to clean rust stains from a chimney, wear heavy rubber gloves to protect your skin from chemical burns or irritation.