Poor service from colourtrade in chatsworth

I bought a 5L can of brick dressing from colourtrade in sunset Avenue in chatsworth in December 2018. I painted my patio slate floor with it. Disaster. White patches appear whenever water, rain settles on it. Complained several times. Told to return can for investigation. Done this. Visited business 6 times already. No response. Now September. Have given up hope of restitution.