Defective paint not refunded by Takealot

I bought paint from Takealot we opened the paint and tested it on a small part of the door frame and didn't use it further after seeing that it's not covering properly as they advertised,I send it back to Takealot because it means it's defective cause it doesn't do the work they promise,And now they refuse to refund me saying I opened the product but how else would I have known that their paint is a bad quality if I had not opened it to test it.

The paint was not used the bucket is still full we just tested it on a small area.