The ‘speckling’ is surely inherent. Spray something very lightly with thousands of tiny droplets and speckling has got to be inevitable. It does make for convincing concrete. The underlying paint, ‘Weathered Concrete’, was ‘wiped over’ with fine abrasive paper before spraying. Since successive coats of the ‘Concrete’ paint had dried to slightly different greys, the light sanding exposed subtly different shades. So this has also helped.
Here’s the same technique used on the coping and lintels (and window-sills) of the warehouse building I finished last year:
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