The best type of sealer to use on flagstone is a water repellent sealer. Water repellent sealers penetrate into the surface of the flagstone where they chemically reacts to form a water repellent barrier. Depending on the type of water repellent sealer, the flagstone will remain unchanged, or it will have a darkened wet look finish.
If you want the flagstone to look as it does now, unsealed, then you want a Silane-Siloxane water repellent sealer. Refer to our Best Water Repellent Sealer article for product suggestions. If you want a wet look sealer for flagstone, something that will enhance the color, then you want something like the WL550. The WL550 is a silicone based water repellent sealer that is designed to darken the surface of the flagstone. It won’t leave behind a surface film, and it will provide years of protection.
If you are looking for a gloss then you need an acrylic sealer but the failure rate of acrylic sealers on flagstone is around 95%. Flagstone is very porous and will allow large amounts of moisture to pass through. Acrylics are sensitive to moisture so when enough moisture build up, it will cause the acrylic to turn white and peel. Acrylics have been used to seal flagstone however so if you want to apply an acrylic sealer to your flagstone, apply the acrylic sealer to a test area first to verify adhesion. Consider applying one light coat instead of two coats so that the acrylic remains as breathable as possible. If you are interested in an acrylic sealer, refer to our Acrylic Sealer Review article for top rated acrylic sealers.