Product Description Marking chalk marks are temporary. Provides greater visibility in standard lighting conditions. Durable formula is weather-resistant and suitable for interior or exterior use. Use to show layout changes and corrections on metal, concrete, lumber, plastic, cardboard and other surfaces. IRWIN Tools STRAIT-LINE Standard Marking Chalk, 5-pound, Blue Chalk (65101ZR) From the Manufacturer The 65101 Strait-Line 5lb Blue Chalk is what you need to help get things Strait. The Strait-Line 2.5lb blue chalk is great for indoor and outdoor use, carpentry, bricklaying, masonary, roofing,carpet installation,floor and wall tiling, painting, decorating and many other uses. Blue chalk is great for temporary lines that will disapear after a while. For more than 30 years, Strait-Line products have represented the standard for excellence and performance in marking tools.