West System 417 Low-Density filler (previously 407) is a blended microballoon-based filler used to make fairing putties that are easy to sand or carve. Reasonably strong on a strength-to-weight ratio. Use West System 417 to extend resin, to reduce weight, shrinkage and sanding. It cures to a dark red/brown colour which is great if you are trying to match it to wood.