There are several important aspects of finishing of a deck, and if stain is being used anywhere on your deck project, wood staining is one of them. A good stain will help protect your wood from sun and moisture damage, and when properly maintained, will add greatly to the life span and looks of your sundeck.
There are many different kinds of stains, and a main division between them is whether the product penetrates the wood, or attaches to it topically like a paint. My preference is a penetrating stain, but of course, there are pros and cons to each. For example, a penetrating stain will be much harder to remove should you choose to change pigment, but in return, offers a much better attachment to wood than a topical type.
When applying a stain, the first consideration is the condition of the wood itself. It must be dry for proper adhesion, regardless of the type of stain used. Ideally, the woodís moisture content should be no greater than ten percent, and less is better. It is worthwhile using a moisture meter to check. A properly cured and dried wood will save labour and materials in the long run. I recommend sound planning that includes either buying dry or kilned wood stock for immediate use, or arranging storage for wet wood and then air drying it. A general rule for air drying is one year per inch of thickness. Use "stickers," squared or rectangular sticks of at least one inch thickness placed at right angles to your deck stock and every three to four feet apart along its length, and keep the weather from it.
Another important consideration is whether your wood deck surface will be smooth or rough. Rough decking means wood that has come directly from the sawmill, and has not been planed smooth using, guess what? ... a mechanical device called a planer. It will not usually be dry enough to stain at this point. Rough deck wood offers a rugged look, and requires only cleaning to remove sawdust and accumulated dirt. If the wood is really dirty, donít buy it, but most accumulation can be removed using a stiff bristled brush (avoid wire brushes unless they have thin, pliable bristles and are used gently). Do not wash it using water. Instead, search for wood washing products at your building supply store, or using the internet. There are several good ones out there.
Most people prefer a smooth deck surface, however. Assuming your wood is dry and seemingly smooth, it is not yet ready for stain, and this is the only disadvantage to planed wood: it needs sanding. This is because of "planer hammer," a state resulting from the planning process that compresses the surface fibres of wood. This compression actually decreases the permeability of wood to coatings, topical or penetrating. Experiment a little on a scrap. An 80 grit sandpaper applied to the wood surface with even pressure will remove compressed surface fibres. To regain the smooth surface that youíre looking for, finish off the sanding process with 120 grit, and continue with 180 grit if youíre really finicky. Remember that this is not a cabinetry finish being applied; it is a surface that will take a beating from deckchairs, feet, and weather.
Note that sanding to remove planer hammer is particularly important on horizontal surfaces, and less so on vertical ones such as railing pales. Overall, these horizontal surfaces such as the deck surface itself and railing tops will bear most of the wearing influences.
Probably one of the most important questions that comes up with staining projects is, How many coats? One coat stains are usually penetrating stains. They usually cost more, but save labour. I usually apply at least two coats of any stain regardless of type, but more is better. Never apply a coat thickly thinking this will save time. Nor do I sand between coats because Iím not going for a cabinetry finish. If you want this, experiment on scraps. It will pay off in terms of looks and longevity of deck coating, but add a lot of labour to your project unless you have specialised sanding equipment on hand. As always, research products thoroughly, ask questions, and once youíve decided on a stain, read the label carefully.
Most of our staining projects are required in areas less than ideal for stain application. Because I take no chances at this important finishing stage, Iíve taken to ensuring dry applications that will guarantee good adhesion, despite what the nearby ocean or rainforest choose to do on a given day. This means using a weatherproof garage, or even setting up an outdoor shed to keep the weather off my projects. Such measures create a short term pain, long term gain scenario, but for me, means keeping to the production schedule. Yes, this means staining a board or two at a time, then, if possible, standing shorter lengths of wood up to dry to save space. Iíve never sprayed stain, believing a brushed-on finish more effective, but many will argue for spray-on applications. This approach makes a lot of sense when staining lathwork, which is a common application, and requires arduous brush time. At present, were I to spray on a stain, I would only do so on vertical surfaces which withstand weather much better.
Finally, we come to the question of whether or not to stain. A properly applied stain protects the wood as mentioned, but if youíre using a wood like heartwood cedar that resists inclement conditions, this consideration is largely an aesthetic one. Uncoated wood will generally turn grey within days. Also, some stains are not "green" products, requiring various processes that negatively affect the environment from manufacture to application and clean up. Fortunately, thanks to the internet, researching products that suit your needs is easier than ever.
Iíve almost forgotten to mention one very important element of getting a good stain coating on wood: some people enjoy doing it. And as is usually the case when pleasure in a job well done is involved, the result will be better. If youíre planning a staining project, find this kind of person and treat them well.