Most manufacturers recommend applying stains and finishes when the temperature is somewhere between 50 F (10 Celsius) and 95 F (35 Celsius). Problem is, they mean surface temperature — not air temperature. Wood absorbs and stores heat from the sun, and the deck surface may be 40 degrees F hotter than the air temperature.
If the surface is too hot, the solvent in the stain will “boil off” before it can penetrate the wood. If the surface is too cold, the finish will become thick and hard to work with.
So do your work in the early morning or early evening, when the deck temperature is closest to air temperature. Of course, this depends on how much sun reaches the deck.
To apply stains, use a brush or a roller. A brush should help the stain penetrate better, especially if the surface is a little rough.
Stains are often fairly runny, so it can get messy. To avoid lap marks work in the shade and complete each board before going onto the next. Let the first coat soak in for 20 minutes to an hour, then apply a second coat. For good penetration, you want to apply your second coat before the first has dried completely.
Try to stain areas that will be tough to get at — like the balusters on a railing — ahead of time. To avoid a mess, one deckbuilder suggests screwing small hooks (like the ones you use to hold coffee cups) into the bottom of balusters and hanging them over a clothesline.
One coat on rough wood will give you 2-4 years before you need to reapply. But a second coat now can give you 10 years of relaxing — rather than working — on your deck.