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I store my gorilla grill upside down as well and it extends the life considerably. I use the original cap and it works fine for me. I place the upside down bottle in a cup so that it doesn't fall on its side.
Have you experienced any delamination issues with the TBII? Some surf board manufactures use XPS foam and there can be delamination issues between the fiberglass and XPS when the board flexes.
Vinegar also works to remove rust. I have used it to clean up rusty nuts and bolts for a restoration project I was working on.