Gaffer tape varies greatly in quality and tack, but it's often a pretty blunt instrument. Some sticks hard enough to pull tufts of pile out of carpet and patches of paint off a stage (I've seen both), some has uneven tack (so paint might get under the edge) and some leaves a sticky residue on hard surfaces. Most is not specifically designed to be waterproof, although some is. Perhaps top quality masking tape might be the answer?