IRWIN Tools STRAIT-LINE Permanent Staining Marking Chalk, Indigo Blue, 4pound (4935524) Great for extreme outdoor conditions because it can withstand harsh weather conditions and months of jobsite wear. Flip-cap keeps the chalk in the bottle where you want it and stops it from spilling out in areas that you don't. Flip-top cap also seals the chalk in an airtight chamber, keeping out moisture and reducing the risk of clumping.