Stains – Crayons, water, smoke and grease can bleed through the topcoat. Prime these areas with a stain-blocking or stain-killing primer. Oil-based stain killers work the best on water stains and for spot priming. Latex stain-blocking primers work better on large areas and hold up better on exterior surfaces. Pigmented shellac primer works well to block smoke and soot damage as well as to block animal urine smells.