I bought a 2001 29 Fever with the commonly used checked flag vinyl graphics. The white part of the flag is cut out so it's gel coat. The boat was used in fresh water and has a bad water line stain. I normally use on/off to remove such stains on plain gel coat. It's really strong and works in seconds. So my question is, will it ruin the vinyl decal where I have to remove the stains inside the cut outs? Is there something different I could use that would work and not damage the decal?

No

The vinyl decal should not be affected by acids.

Its not a decal.

its paint, before 1995 imron after PPG

Just do NOT get it on any galvanized trailer, lift or wheels. It will take off the galvanized coating.

Zep toilet bowel gel put it on let it sit wipe it off

https://www.booyahclean.com/products...t-hull-cleaner

Go to amazon and buy some slimy grimy. Its safer on your trailer and not expensive. Works just like all the other cleaners discussed.

ON/OFF