Using a sealant on the inside of a wooden urban planter box is a common idea to prolong its life, but it requires careful execution. The constant moisture from soil and water can cause wood to rot quickly. While sealing can create a protective barrier, it's not a simple yes-or-no solution.
The primary goal is to prevent rot without harming your plants. For this, you must use a non-toxic, waterproof sealant specifically designed for garden use, such as a pond liner, a food-safe container sealant, or a specialized planter box liner. Never use standard deck sealants or those containing chemicals like creosote inside the planter, as toxins can leach into the soil and damage plant roots.
The most critical step is ensuring proper drainage. Sealing the entire interior, including the bottom, without drainage holes will create a swampy environment that drowns roots and accelerates rot. The correct method is to seal the side and corner seams thoroughly, then drill adequate drainage holes in the bottom. You can also consider lining the interior with a physical barrier like heavy-duty plastic (with holes for drainage) before adding soil, which is often more effective than a liquid sealant alone.
In summary, you can use a safe, appropriate sealant on the inside of a wooden planter, but it must be paired with excellent drainage. A combination of sealing the wood's seams and using a separate liner is often the best strategy for a planter that lasts for seasons to come.