Chaff is to radars what a smoke screen is to the human eyeball. It is not easy to control, it obscures whatever it surrounds and prohibits any useful observation of what it is used around. You have correctly stated that it is made up of fine, hair like particles coated with radar reflective material. Those fine particles disperse in a cloud and drift with the wind. Chaff clouds are completely at the mercy of the wind, and any chaff used anywhere near TWA800 would have shown up as a large 17-19 knot track (the speed of the wind at the altitude TWA 800 blew up). You cannot "paint" something with chaff any more than you can "paint" something with smoke. A kinetic kill vehicle is designed to be used above 100,000 feet. It is an incredibly complex device that has undergone years of testing. To mark its impact with a cloud of chaff would be like marking a point of brain surgery with a paint bomb. You would obliterate any useful info from the impact, and as I've previously stated, both the warhead and the target are already being tracked by other sensors.