What is 'blowhole' filling?

Blowholes are small, pinhead-sized voids on the surface of concrete caused by trapped air bubbles against the shuttering during casting. While structurally harmless, they look poor and can harbor dirt or moss. We fill blowholes using a fine 'fairing coat' or cosmetic mortar. This is scraped tight across the surface to fill the holes, providing a smooth, uniform finish ready for painting or coating.