We always recommend doing a mock up. You will be able to evaluate aesthetics, coverage rates, and performance.
How To: Affix a RILEM Tube to substrate for testing

To ensure that you get a true test, it is important that the RILEM tube is properly attached to the area you wish to test. If you don’t get a good seal between the putty and the wall, you might get a false fail (leaking through the putty). Note: Ensure that the inner opening is not obstructed with putty. Putty is resusable. Store in a cool place. ...