Freepers might want to look into something called "Liquid Metal Embrittlement" agent. It can be put into a felt tip pen, and when applied it changes the metal structure, making it brittle, weak, and likely to fail from normal operating loads. Potentially a dangerous sabotage weapon. Search the net and you can find articles, including one published in The Futurist in 1989 (sorry I don't have a link) specifically on the subject of terrorism.
From http://www.wfs.org/cetron89.htm
Reprinted from THE FUTURIST, July-August 1989
The Growing Threat of Terrorism
By Marvin J. Cetron
Chemical and biological weapons. Chemical weapons range from old-fashioned poison in the water and nerve gas to a new Liquid Metal Embrittlement agent (LME). Applied with a felt-tip type pen, LME is a clear, invisible substance that changes the chemical structure of a metal so that it is no longer resilient and flexible. The result: The metal can fracture under stress. Trucks, airplanes, or bridges would be vulnerable to catastrophic failure without advance warning...