Our family is trying to repair our deck. Prior to any sort of staining, of course, it was necessary to clean it of first. Afterall, the entirety of the structure was gray, and now it actually looks like wood again.
However, we are concerned that the 6 h.p. powerwasher used was not good enough to allow the staining the longest possible life. A picture is here:

The black bits seem to be present just at the location pictured, however, you'll notice there are still streaks of grey on the newly revealed wood color. What I'm worried about is that discoloration indicates places where the new sealant is more likely to fail than it should be due to dirtiness on application.
The service was done by a subcontractor, but the overall project was expensive enough I think we can have high standards for this. Should I be concerned at all about what I"m looking at if this is a done job? I'd appreciate any advice.
(Also, he appears to have forgotten an entire rail.)