3M™ Transparent Process Colour Thinner, 991 is a thinning agent intended to be mixed with 3M™ Transparent Process Colours Series 990 Inks. This colour thinner helps prevent premature drying of inks when screen processing copy or background colours on 3M™ Engineer Grade Prismatic Reflective Sheeting Series 3430, 3M™ High Intensity Prismatic Grade Reflective Sheeting Series 3930 and 3M™ Diamond Grade™ DG³™ Reflective Sheeting Series 4000.