I would try the following:
1) Scour the trough with steel wool, to loosen all particles.
2) Brush the loosened particles to one end of the trough.
3) Use a vacuum (especially with a narrow attachment!) to get out as many particles as you can. You can also sweep / brush the particles to the end again, and vacuum them out again.
4) To remove the final particles, use an air blower such as a can of compressed air or the output stream from a vacuum. You should probably be running the vacuum on the same area, so the particles get sucked up instead of flying all over the place.