I heard a funny deck story from a client last week. It's summed up with this quote from him, "I should've just handed him [13 yr old son] $250 and done it myself." There were swirls all over the deck from power washing with too much pressure, stain on the vinyl siding up top, and stain on blue stone from lack of drip protection. This week another client told me about how her father-in-law power washed and stained their deck while they were out of town a few years back. Her comment, "There is stain on the siding, but can you tell Dad he did a bad job? I guess we got what we paid for..."
Adults only - This is not your kid's summer job. Find something else to get him on cruise control for the summer.
Be careful with the power washer - If you see splinters on your legs when power washing you're using too much pressure. Use a fan tip, keep the tip moving, stay a consistent distance from the surface.
Cover up - Use drop cloths and painters tape to make sure you don't stain the siding or allow it to drip on the siding or patio below the deck. It's an outdoor room so do the same protection you would on an inside project. Wear rubber/latex gloves.
Don't buy cheap stain/finish - I recommend buying a stain or finish that feels expensive ($40/gal)
Expect bi-annual maintenance - Every other year you should expect to restain your treated or cedar deck. If the deck is already in less than perfect shape, expect annual maintenance to keep it at its handicap.
Outcome: If you follow the ABCs your wood treated wood or cedar deck will still be looking great 15 or more years from now. If you don't, it'll look junky in <5 years.
Staining- With stain you can only keep it the same color or go darker. You cannot change the color to be lighter than it is now. The more pigment in a stain the more it acts like a paint. Paint is a coating that sits on the surface of wood. It breaks down quickly as wood naturally expands and contracts through water absorption. Stain penetrates. I prefer a penetrating stain over a paint or natural deck sealer. Cheap, paraffin deck sealers are a waste of money in my opinion.
Power washing - I recommend you rent a power washer versus owning one. If you must own one, you can probably just own an electric power washer. I power washed my siding 2 weeks ago and spent 2 hours trying to fix my dad's power washer and less than an hour washing the siding.
Pricing - Expect to pay $700-1200 for a pro to power wash and stain your deck.