Mineral Staining on Texture Coatings

Not all brown spot staining on a rendered, textured, or painted surface is from metal rusting. There are forms of algae and pollen that occur naturally called “coffee rock” that also presents as a brown stain.

Rust stains can be caused by welding and/or metal grinding in the vicinity of the textured coating. Iron-containing metal particles can become airborne and become trapped on the soft acrylic surface. Mineral staining then often occurs quite rapidly, within a few weeks and often within days for building surfaces near the ocean.

The presence of contamination with metal-containing material is usually betrayed by vertical “tails” (coloured stains) going down from the mineral stain spot from 1cm to 5cm in length. These stains are usually brown but can also have a yellowish shade.

The mineral staining is purely aesthetic and does not affect the performance and durability of the coating system and once the offending particle has been removed there is no chance of the stain recurring.

The wall colour is a lot easier to touch up with Armour in a reproduction of the original colour match if the problem is caught before any fade or weathering has occurred.

Procedure for the rectification of Mineral Staining on Paint and Texture Coatings

  1. Remove the stained particle. This can be done with a sharp pointed object, tweezers, or the tip of a knife. If the rust stain is caused by coffee rock the particle should be a soft, dark powdery substance.

    It is important that the source object is removed to prevent future rust stains.
  2. Using a soft nylon brush (i.e., toothbrush) scrub to remove the stain or reduce its appearance as much as possible with a mild Oxalic Acid solution such as Diggers Rust and Stain Cleaner. The solution should be milder than the specification; ROCKCOTE recommends a 10% solution in warm water (i.e., 100 grams per 1Lt warm water). Use a slightly damp sponge under the stain to prevent the Oxalic solution spreading down from under the stain.
  3. Flush the treated wall area with liberal quantities of clean water. Prolonged contact with Oxalic acid cleaner may result in bleaching of the paint and/or deterioration of the paint film.
  4. Once the surface is dry and at a moisture level of less than 15% WME, the hole created by removal of the rust particle can be filled with Armour (small holes) or Polymer Render/Sikaflex (large holes).
  5. Once the material in the hole is dry (approximately 2 hour), then touch up the rust spot hole and the cleaned rust tail (if required) with touch-up paint or the correct colour in Armour.
  6. If the wall colour is too old or faded for an “invisible rectification” then it may be necessary to repaint the entire wall surface. It is advisable to use a lower sheen level (i.e., flat instead of low sheen) if the wall is 6 months to 2 years old.