I have a typical older GB ... gray decks, protruding seam sealer, popping bungs, etc. My advice is to only clean it gently and leave the rest to Mother Nature.
1st story. 5-6 years ago I was knocked down by an unexpected wave offshore while hauling an anchor and hit my head very hard on, I think, the forward cabin hatch. The gash on my head was significant and while I was out cold for a minute or two blood pooled nicely on the forward deck (TMI?). The stain was not able to be cleaned off with anything I'd use on the old decks so I left it. The sun, rain, and salt spray slowly faded it away so that two years later it was effectively gone. I don't have any pics or I'd share them.
2nd story. In 2017 we left our boat on a mooring for a few days while we went home to mow the lawn, pay a few bills, etc. When we returned & went out to the boat we found that an osprey or two had tried to build a nest on top of the radar antenna. Needless to say, there was one giant mess on board including massive amounts of bird turd on the flybridge cover, the flybridge deck, the nicely varnished hand rails, and several large sticks & twigs spread around the flybridge, the bridge steps and both side decks. I moved the boat into a slip and attempted a clean up but of course the turds did not come very clean. Me not being very fastidious and with the weather not cooperating for a pleasant cleaning experience I decided to once again leave Mother Nature to take care of the remnants of the problem. This took only a couple months for all the stains to disappear.
There just my ten cents on this subject.