Cleaner and brightener didnt remove black spots - pressure wash?

You need to pressure wash off after using the cleaner. The oxidation/graying will come off and so will the mold. Brighten all after.

Definitely prep better before applying the stain. You can try a store bought cleaner but look for one that is a powder.

Thanks! Looks like Home Depot has a gas pressure washer 2000-2700 psi or an electric one 1400 psi. I want the electric one, right? And see if I can lower the pressure a little? What is the optimal setting?

Thanks so much for the quick reply! I feel more optimistic having a plan. I was able to get more Defy cleaner and brightener from Amazon and by paying extra I can get both by tomorrow. So I hope to finish the prep this weekend after all, and then there might be 2 sunny days in a row next week.

Is there an optimal time of day to stain? Afternoon when the wood's at its driest, after the morning dew has evaporated off?

Use the gas one. You do not lower the pressure, just wash from farther away. For example a 2700 psi pressure washer will wash at 2700 psi it you are 1/10 of an inch away. If you are 8-12 inches away from the wood, pressure will drop in half roughly.

Do not stain in direct sunlight. Better early in day or later afternoon.

I repeated the cleaner and brightener, pressure washing in between, and I was able to get the gray off, though the mold mostly just lightened. (The pressure washer wouldn't restart after I refilled the gas, so I scrubbed the rest of the rails and the decking with a stiff brush or just the spray nozzle on the hose, which made it easier to be thorough without risking getting too close. My problem before was definitely due to trying not to remove material, instead of understanding that the point is to remove the gray wood. Thanks for explaining that.)

The deck now looks bright and new, but with lots of fuzzies. I'm sanding with 80 grit to get them off (using instructions from http://www.deckstainhelp.com/wood-fuzzies-furring-on-deck/ ), and I have a question about that.

In many places, the amount of gray removed left behind raised grain. Should I sand that down to a smooth surface? What grit sandpaper would be good for that?

First image is before and after of removing fuzzies on a rail with 80 grit. Second image is of the raised grain. Third image shows how the fuzzies are down between the raised grain, so I might need to sand it all the way down to get rid of them.

Much easier to get a rotary buffer and use sanding pads. If you use paper, use 60-80 grit. You do not have to remove all the fuzzies as they will darken when you apply the stain.

After a bunch of rain we finally had several sunny days in a row, which was helpful because the remaining steps took a lot longer than I expected. I sanded everything with a mouse sander because it was what I had and there was a lot of sanding to do on the rails. That took maybe 6 hours, not too bad. I was able to improve the raised grain areas and get off a lot of the fuzzies. Not all the mold came off but it lightened. Since it had been over 2 weeks, I cleaned and brightened again afterward.

My husband and I started staining the next day and it took about 20 person-hours over 4 days, most of it on the rails. I'm happy with how it turned out, but the deck boards are uneven in color and I'm curious why.

Photo 1 below shows post-sanding and post-staining. Photo 2 shows how water beaded up on the areas with deep color but not on the areas with light color. I assume that means the stain didn't penetrate well. (Photo 3 shows how the rails looked after sanding, just as a follow-up to my previous posts.)

We did the deck in 2 portions to meet the 20-minute wait time between coats. The second portion has uneven color and the first (closer to the house) looks pretty even. Here are possible differences:
1. We traded jobs of using the car wash brush and cutting in the edges - my husband brushed the boards on the first half, and me on the second half.
2. The timing might not have been exactly 20 minutes on each - I'd guess longer on the first half but I can't remember.
3. The second half needed more sanding than the first. (I used 80-grit sandpaper and I did clean and brighten again after.)
4. The second half looked to me like it might still have more fuzzies left than the first half.

Any insight as to why this happened? And should I try applying a third coat on those lighter boards now (likely to penetrate since water isn't beading up)?

Looks like the stain absorbed really well in those areas. Leave alone. In the Spring, lightly wash to remove any dirt and apply a light coat over the entire deck.

Fantastic. Thank you for all the help and for running such a great resource in this website and forum!

Got the same problem, previous replies were helpful.. This is a gem, thank you!