The seascape shown here (along with a small detail) is a painting where I obliterated much of the color of the hills and sea with a velantura of titanium white. I diluted the white acrylic with water and glazing medium, then worked fast with a big brush. As titanium white is such an opaque pigment the effect is dramatic and immediate. I always keep a damp cloth to hand in case it's too much! And remember you can use pressure to spread the opaque paint thinly across a surface, you don't need to thin the color with a glazing medium or oil/turps.
"Seascape No.9" © Marion Boddy-Evans. 46 x 122cm. Photo licensed to About.com, Inc