Secondly are the bulking agents which reduce the density of the putty and make it easy to sand. These include glass microspheres, phenolic microspheres and wood flour.
Wood flour is also a thickener.
Lastly are milled fibers, which add a matrix of glass fibers to the mix. These are used to add structural tensile strength. This may be varied depending on the thickness and desired density. The microspheres will thicken the resin but will result in a compound that will sag on a vertical surface. Only the thixotropic agents allow for non-sag mixtures.