For a natural pine "butcher block" kitchen island countertop, using an appropriate wood finish is essential to protect the wood and enhance its beauty. The choice of finish depends on your preferences and the level of protection you desire. Here's a brief overview of the three options you mentioned: oils, polyurethane, and shellac.
Oils (e.g., Mineral Oil, Tung Oil, Linseed Oil): Oiling the wood is a common and traditional way to finish butcher block countertops. Mineral oil is food-safe and can be used to condition the wood, enhancing its natural color and grain. Tung oil and linseed oil are also options, but they might take longer to dry and cure. Oiling creates a natural, warm, and matte finish. The downside is that it requires more frequent reapplication, especially in a kitchen where water and spills are common. Additionally, oils do not provide the same level of protection against scratches and stains compared to other finishes.
Polyurethane: Polyurethane is a durable and water-resistant finish that creates a protective layer on the wood surface. It's available in both water-based and oil-based varieties. Water-based polyurethane dries faster, has less odor, and provides a clear, non-yellowing finish, which is particularly important for maintaining the natural appearance of pine. Oil-based polyurethane might impart a slight amber hue to the wood over time. Polyurethane offers excellent protection against water, stains, and scratches, making it suitable for high-traffic kitchen areas. However, keep in mind that it creates a more "plastic" or glossy appearance compared to oils.
Shellac: Shellac is a natural resin derived from insects and is often used as a finish for woodworking. It provides a warm and natural look to the wood while offering moderate protection against moisture and stains. However, shellac is less durable than polyurethane and might require more frequent reapplication in a kitchen environment. It also tends to be more sensitive to alcohol and certain chemicals, so spills should be cleaned up promptly.
Considering the nature of a kitchen island countertop, which is subject to spills, water, and food preparation, polyurethane might be the most practical and durable choice. Water-based polyurethane, in particular, will preserve the natural appearance of the pine while offering strong protection against daily wear and tear. Just make sure to follow the manufacturer's instructions for application and drying times. Whichever finish you choose, always apply it in a well-ventilated area and follow safety precautions.