Hard to tell in the picture what you got or how much they sanded. I just restored a 20 year old deck in a house we bought. It came out gorgeous considering what it looked like originally. There are many cleaning products out there. After much reading I used the Restore-a-Deck system ( http://www.restore-a-deck.com/Order-Restore-A-Deck.html ). It worked great. It's a two step process. A cleaner and a brightener/neutralizer.

here is the finished deck, The stain is Armstrong-Clark semi-solid Woodland

This post was edited by splais on Wed, Jul 23, 14 at 8:03

Congrats! your deck looks great! The more I ponder, I'm thinking they just didn't sand WITH the grain on those boards. There were 2 people, and maybe one was new to sanding? I attached a closer view here. Before I ask them to re-sand those boards, I'd love to know from more experienced people if my diagnosis is probably right or not!

Looks like they pressure washed.

There is no "with the grain" when you're using a power sander.

You should use the deck cleaner and brightener, mentioned above, before the application of a new coat of stain.

I'd also recommend a penetrating oil type of stain rather than a semi-transparent water based pigment. The oil-based ones are easy to reapply and touch up later on down the road. The water based stains require the whole sanding down to bare wood, basically a complete strip, when you need to do a new coat.

Thanks much Aidan. They power sanded with a small tool (on their hands and knees). Then they removed the dust and power washed with plain water (they said they use low pressure when I asked if washing after sanding would raise the grain). The deck had lots of cracks and splinters, so it's much smoother now. But, they did not use a cleaner or brightener. I attached a "before picture". Question: why is a cleaner necessary if the whole surface has been sanded? Thanks again for your help - REALLY appreciate help on this!

This picture looks like the deck was power washed and perhaps sanded but not sanded well enough.

Thanks Mike. Based on feedback from you and others on this forum, and further research, I think the blotches are just left over stain. Rather than further sanding, I plan to strip and brighten and then apply Defy Extreme Stain. It's a newer water based that is NOT film forming.

Just a side note. We accidentally spilled some CLR on a graying teak table, and noticed that it brought that old teak back to new! Keep it in mind for other wood. We have since learned it's just the best!

i think what you are seeing there is the strokes they either did with a power washer or with a sander.

When a sander or washer goes in one direction, then stops and moves back, the time that is used in the "stopped" position causes that part of the deck to be more sanded/washed. So the lighter areas you see is a part in the process that became cleaner/more sanded because it was gone over in quick succession. Make sense?

The same is true if you are painting anything. That's why the pros always (if they can) have a continuous stroke from top to bottom or side to side. If you ever painted a multi-panel door, you would know what i mean.

Thanks tjdabomb... that makes sense. I'm planning to apply a stripper, then a brightener. Do you think that'll remove these blotches?

No, because if it is improper sanding the entire thing needs to be properly sanded. Pressure washing more than likely the same scenario. More than likely the only way is to start over or live with the "blotches" as you call them, however remember any finish will look the same only colored differently.

bjindc, that might work or just highlight the errors you aready see.

if you get a sander, get the right paper and TEST on one inconspicuous board - one near the end. From there, sand in long movement and not short choppy movements.

you basically need to "clean" the areas left dark by the deck board areas not hit twice in short strokes whern it was originally cleaned. It will take time but be ok in the end, I promise!