Black / gray deck spots - how to fix / prevent?
These weird and hard to clean spots have formed on my deck. They form in random patches. Please see these photos:
It's really hard to get them off - lots of scrubbing that I'm sure damages what's left of my non-existent gelcoat.
Does anyone have any idea what causes this? What I can do to prevent them? Also, any good ideas on how to clean it other than scrubbing and FSR?