Whether to brush or roll stain on wood depends on the project and your preference. Brushing is ideal for smaller, intricate areas and offers more control over the application, ensuring thorough coverage and penetration into the wood's grain. 

Conversely, rolling is efficient for more extensive, flat surfaces and can save time. However, it is not always the best fit for textured or uneven wood. Some projects may benefit from combining both methods, using a brush for detail work and a roller for broader areas. Ultimately, the choice between brushing and rolling depends on the specific requirements of your staining project.

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. 

Limewash and whitewash do not harm surfaces if you apply them correctly. Use on non-suitable surfaces can cause issues. For instance, applying limewash or whitewash to surfaces with existing sealants or incompatible materials may lead to poor adhesion or flaking. Also, harsh cleaning agents on limewashed or whitewashed surfaces can damage the finish. Proper preparation, application, and maintenance ensure that limewash and whitewash do not harm surfaces.

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.