Preparing a deck for a coat of stain is not a difficult job.
I am not a fan of using a power washer as the main tool for this job. The high pressure water tends to remove more pulp wood than grain as one is cleaning the surface.
With products like OXI MAGIC, OXI-CLEAN and other oxygen bleach products the job can be safe and easy. It won’t hurt you or your plants like my OLD favorite of TSP and liquid bleach can. Mix it according to the strongest directions (start with 2-scoops, or so, per ½ gallon of warm water) and apply the mixture to the deck.
Apply it to a dampened area of your deck with a scrub brush that attaches to a broom handle. This will keep you on your feet and allow you to cover a larger area quicker.
Cover an area no larger than what you can do in about 10-minutes. I don’t want it to dry out before I apply a second light coat over the first coat. You should see the dirt and algae running for mercy. Once the dirt is loose, you rinse with a garden hose and move on the next section of your deck.
For dark stains that oxygen bleach can’t get out, mix a concentrated batch of oxalic acid, also known as wood bleach. It is usually used to take water stains out of hardwood flooring. I mix small amounts, about a pint, in hot water and I stir in as much powder as will dissolve. Then using a paint brush I apply it to the stained areas and let it sit 5-minutes. The stain should liquefy and be easily rinsed off although some stains require a second coat.
The use of a power washer should not be required. When using one on a deck, be careful not to ruin the deck by blowing off all the soft pulpwood. In most cases the use of a power washer on a deck should be left to a professional. If you do use one be sure to use a wide tip and don’t hold it close to the deck.
If the wood looks good wet after you rinse it off, it will probably look good with a deck sealer on it. And remember – do not apply a deck stain to damp wood! Sealing in moisture is guaranteed to ruin most any exterior finish.