I just use food colouring (from local supermarket) with naturalTimbermate... not white, as that "pastels" the colours. The Timbermate sands beautifully... the only catch is to pat it down and not pull it across, as if your engraving has little "pilons" sticking up through the surrounding engrave (engraved away), pulling the filler across may break those pilons.
Still the red comes out more pinkish, so I use Chinese red food colouring (powder)...
I know I gave the specs somewhere but it's 1:30am 1:45am and I'm gonna go to bed.
If your first cut is too short... Take the second cut from the longer end... LBD
