While it's tempting to use leftover house paint to seal the inside of a wooden planter box, it is generally not recommended. The primary concern is toxicity. Most standard house paints contain chemicals, fungicides, and volatile organic compounds (VOCs) that can leach into the soil. These substances may then be absorbed by plant roots, potentially harming or even killing your vegetables, herbs, or flowers. For edible plants, this poses a direct health risk.
Furthermore, house paint is formulated for rigid, stable surfaces like walls. The constant moisture, soil acidity, and temperature fluctuations inside a planter will cause the paint film to crack and degrade quickly. This compromises the seal, leading to accelerated wood rot and defeating the purpose of sealing.
For a safe and effective seal, use products designed for garden use. Excellent, non-toxic options include:
* Raw or Boiled Linseed Oil: A natural, penetrating oil that protects wood from within.
* Tung Oil: Provides a durable, water-resistant finish.
* Eco-Friendly Wood Sealants: Look for plant-based or "green" sealants labeled safe for garden beds.
* Pond Liner or Plastic Sheeting: As a physical barrier, line the interior before adding soil.
Always allow any sealant to cure completely—for several days to a week—before planting. This ensures off-gassing is complete and creates a healthier environment for your plants to thrive.