Use Cross Flood Fill 
or Pattern Flood Fill 
to fill 
an area with crosses or a fill pattern. An empty area can be flood filled until it meets existing crosses and/or backstitch. A connected area of the same colored crosses can be flood filled until it meets an empty area, backstitch or crosses in a different color. Crosses are not considered to be connected for flood fill if they only meet at a corner.
When an area is flood filled, the fill does not relate to the background picture. Sometimes it may help to hide the background picture so that you can clearly see the area you wish to flood fill; for example, if there are lines in the background picture that are similar to backstitch.
In some cases, the fill will not go through narrow areas. If the fill could not continuously connect a fill of full crosses through a narrow gap, the gap will act as a barrier to flood fill.