Gouges and holes in wood are more difficult to repair than dents because wood fibres have been cut, torn and removed from the wood. Larger gouges and splits are best repaired by patching the void with a piece of wood that matches the colour and grain orientation of the wood under repair. Patching wood requires skill, but when done properly it is possible to create a repair that is very difficult to see. An alternative to patching is filling (sometimes known as stopping). Numerous coloured fillers (putties and waxes) are produced commercially and are coloured to match different wood species. Successful filling of voids in wood requires the filler to precisely match the colour and grain pattern of the wood around the void, which is difficult to achieve in practice. Furthermore, filled voids do not behave like wood during subsequent finishing steps, and they age differently to wood. Hence, repairs to wood using fillers may noticeable. Therefore filling is best used with opaque finishes rather than semitransparent finishes, which allow the grain of the wood to be seen.