Ultimately, overapplication of stain can compromise the deck's appearance, durability, and longevity. It may result in an uneven finish, with excess stain pooling, dripping, or drying into thick patches. It can also obscure the wood's natural grain and texture, detracting from the desired aesthetic. Moreover, excessive staining may not fully penetrate the wood, leading to poor adhesion and premature wear. Additionally, thick layers of stain are prone to peeling, cracking, and blistering as they struggle to bond with the surface. 

While experienced DIYers may take on the challenge, hiring a licensed professional is often the best route, especially for larger or elevated structures. A local deck contractor is typically the best pro for the job, as they have specialized experience and are knowledgeable about local building codes, ensuring your deck is built safely and designed to last. Other professionals who can build a deck include general contractors and carpenters, who may be more readily available in some regions.

If you want to make a deck cleaner from products already in your home, there are various options. The most common is to use a gentle blend of 1 gallon of warm water and 2 tablespoons of ammonia-free dish soap. You can also mix water with distilled vinegar. Sprinkling on some baking soda can help to remove mold and mildew. You can add oxygen bleach (but never damaging chlorine bleach) to the water and ammonia-free dish soap solution for something a bit stronger. 

It is definitely worth sanding a deck to ensure the wood absorbs new stain. Sanding a deck removes the outer layers of aging wood, allowing the stain to penetrate. It also gives a fresh look to the entire surface as part of the deck refinishing process. Without sanding, your stain or sealant is less likely to absorb into the wood. While sanding a deck is a time-intensive project, the rewards are noticeable and long-lasting.