trees? as in oak trees
not sure how long it was parked but if it was oak leaves falling
on the boat
then your dealing with tanic acid which also stains cement....
i'd try chlorine bleach diluted with water and some dish soap...
start weak ....go stronger as needed.
dont use pool chlorine as thats 90% pure
and laundry bleach is @ 10 % concentration
lemon juice will bleach stains out too.
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