Outdoor Additive | Knot Blocker

Outdoor Additive | Knot Blocker

$10.99 USD

Add to milk paint to enhance any paint job for exterior use. Outdoor Additive boasts of its ability to reduce tannin’s leaching through milk paint from wood knots and discoloring the surface. It also provides improved resistance to mold and mildew growth. Topcoat recommended, such as Outdoor Defense Oil, for water repellency.

UPC: 813292020539 SKU: ODA Category:

Product Description


“Outdoor Additive” by The Real Milk Paint Company is an exterior paint additive made with borate compound, similar to borax. Borax can be found at the grocery store as Twenty Mule Laundry Booster. It is mined from the ground mainly in California. Borax is safe and non-toxic and has multiple uses. “Outdoor Additive” is a slightly modified borax compound that is very finely milled, so that when added to the painting furniture project it remains smooth.

Real Milk Paint® with “Outdoor Additive” is suitable for exterior porous surfaces such as wood, concrete, brick, and stucco. It should not be used over existing paints. The paint would need to be removed first. Real Milk Paint® with “Outdoor Additive” will not blister and peel like modern paint but will act as a sacrificial coating that will gently wear away with time. The way paints are supposed to work.

Now Real Milk Paint® can be used on your outdoor projects. Having an old house built in 1892 with original wood clapboard siding and windows, I wanted to use a period-appropriate paint for the restoration. While milk paint does work well outdoors as is, adding “Outdoor Additive” makes the paint even more superior and is one of the best wood finishing products that you can use.

For enhanced exterior use and improved water resistance, topcoat Real Milk Paint® with Outdoor Defense Oil.


  • Tannin Stain Blocker – blocks stains from coming through the painted wood such as cedar and knotty pine.
  • U.V. light protection – inhibits ultraviolet light from destabilizing the paint film.
  • Mold Inhibitor – inhibits mold from developing (used at the recommended level).
  • Flame Retardant – helps to control and reduce the spread of flames.
  • Bactericide – will not promote the growth of bacteria in liquid paint.


Package contains 1 cup of Outdoor Additive.

  1. Mix 1 gallon of Real Milk Paint with water as per the directions on the label
  2. Add this entire container of Outdoor Additive to the Gallon of Real Milk Paint (contains 1 cup of powder)
  3. When dry, there may be some chalky look. This will wear off the painted surface as it weathers.
  4. Apply a topcoat such as Outdoor Defense Oil over the painted wood for best results.

To mix smaller quantities :

  • 1/4 cup per Quart of Real Milk Paint
  • 1/8 cup per Pint of Real Milk Paint

Video Instructions

Download the Outdoor Additive Safety Data Sheet by clicking here.