Grain fillers generally consist of three basic components; a binder, a bulking agent and a solvent.
[1] The binder is wood finish, and in the case of oil-based fillers is typically a blend of oil and varnish.
Both types of filler use silica as a bulking agent as it resists shrinking and swelling in response to changes in temperature and humidity.
[2][3] Other bulking agents may include quartz powder, wood flour, and talc.
[3] Woods such as ash, oak, mahogany and walnut fall into this category, while tight grained woods such as maple and cherry can achieve a full finish with just a few coats of finish.