Do I need to sand after stripping
You'll need to neutralize the stripper with water. The water will raise the grain of the wood so you'll sand to remove the raised grain.
Protect the deck as much as you can. There's a good chance you'll need to wash off any stripper splash and resand to remove any raised grain.
The gray areas can be additionally sanded or the piece replaced.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
B. Franklin 1759