No, not a guitar...
I am filling 6 fenders with water, so they are as negatively buoyant as possible. I bought really heavy duty ones that have dense plastic, that I reckoned would sink. I bought them un-inflated, but they obviously don't arrive completely flat, because when pressurised with water they still have some air in. You can hear the water sloshing about; quite a bit of air in there. I thought it would be a simple task of pressing the valve when it was positioned uppermost and the pressure in the fender would expel the air.
Except it doesn't. Water comes out. I have rolled the fender around and cannot get air to come out. If I shake it violently from side to side, an occasional fart appears but it's mostly water. So it looks like the valve has a long tube on the back - though I don't know why it should have.
When filled with water and in the bath it is a kg or so positively buoyant and I reckon with the last of the air expelled I will reach my goal of negativity.
Of course I could weigh it down when underwater, but that's not too elegant.
Anyone seen a fender that's exposing its innards and care to say why no air is coming out? Nothing obvious from googling their design. They are Majoni fenders for what it's worth and have the sort of valve that you use a football spike type inflator on.
And if anyone is really interested I'll describe why I want some negatively buoyant fenders.....