Duct Tape is a strong, heavy tape reinforced with cotton fibers. It can be used for a variety of purposes in foamsmithing. Its most common uses at this point are for reinforcing the flat of a sword blade (particularly near the tip), in pommel construction (where its weight helps counterbalance the weapon), and on shields. Duct tape should only rarely and minimally cross a striking surface, if at all - duct tape along a striking surface will make a weapon 'slap', or cause the weapon to sting unnecessarily.
Duct tape is also commonly used to cover incidental padding, cross guards, and pommels. The variety of available colors can make it an attractive option. It does wear with age, however, and will become less attractive over time faster than Electrical Tape or hockey tape.
Poor quality duct tape should be avoided - duct tape should be obviously thick and strong, and while the cotton threads should be present, the tape should feel smooth to the touch. Gorilla Tape is popular in foamsmithing due to its high quality.
Duct tape is also commonly used in shield construction, where slap is not a concern.
Duct tape is available in a wide variety of strengths, levels of adhesion, and general quality - bad duct tape is not worth using. One consideration is how well the tape tears down its length. If it tears cleanly into half or third strips, it is good duct tape.