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.

Many decking oils can help slow down algae growth, but they don't completely stop it. These oils work by sealing the wood, which reduces moisture and, therefore, algae growth. Some decking oils also help protect against UV damage, but they need to be reapplied according to the manufacturer’s instructions to ensure the decking stays protected.

There are more pros than cons when staining a deck. Deck stain is more affordable than paint, it protects the wood and allows the natural textures to shine through, and it can revive the appearance of your home. However, deck stain also requires prep work (sometimes days or more), carries more upkeep than a painted deck, and can fade over time if not cared for. 

Hiring a contractor or professional deck refinisher is the best way to ensure your deck is properly stained. They have the expertise, tools, and supplies to do the job quickly and efficiently. However, staining a deck is a manageable task for most homeowners, and doing the job yourself can save money. Just keep in mind that DIY staining is a long process requiring physical strain, so it may not be the best option for everyone.