Marine Boat Seats How Do I Get Black (mildew?) Spots Out Of White Upholstery In My Boat Seats? I've Tried Bleach Based Cleaners

How do I get black (mildew?) spots out of white upholstery in my boat seats? I've tried bleach based cleaners - marine boat seats

I tried to bleach based cleaners, but do not want to take them for a long time, because it weakens the material. I suspect they are there and dusty. Displayed while the boat is stored at the output. It is the only thing that can soak it and is expected to penetrate the tissue? The fabric is marine grade vinyl, and I even used a pressure washer 3000 PSI, with current products. All proposals which do not deteriorate to see my seats would be greatly appreciated. Thanks