On Tuesday, October 23, 2012 11:28:40 AM UTC+1, wrote:

You can get slow-closing solenoid valve for this application.

You could also use a solenoid valvle as a pilot valve operating a diaphragm valve, with a restriction in the diaphragm discharge; I can't recall how that's done now. Many large 'solenoid' valves are pilot operated diaphragm valves.

Plan B, get a motorized ball valve; less likely the stick open than a solenoid.