To keep a pumpkin from rotting, there are several methods you can use. One of the most popular methods is to spray the pumpkin with a bleach solution. Here are the steps to follow:

1. Clean the pumpkin: Remove any dirt or mud from the pumpkin using a damp cloth.

2. Mix the solution: Mix 1 part bleach with 3 parts water. You can pour the solution into a spray bottle or a bowl/bucket and dip a rag into it.

3. Spray the pumpkin: Spray the entire surface of the pumpkin with the bleach/water solution or dampen your cloth in the bleach mixture and wipe down the entire shell of the pumpkin thoroughly. Pay special attention to making sure the stem and the blossom spot on the bottom are both sprayed with bleach solution.

4. Let it dry: Let the pumpkin dry completely before proceeding to the next step.

5. Seal with a clear spray paint: Spray the pumpkin with an acrylic spray, which is a moisture-resistant adhesive that will keep your pumpkin from rotting or getting moldy.

Other methods to preserve a pumpkin include using petroleum jelly, refrigerating it, soaking it in water overnight, or coating it with WD-40. However, spraying the pumpkin with a bleach solution is a widely used and effective method to keep it from rotting.