ok, what you have described is not what volunteering is all about, volunteering should be you working in a role that helps them achive the company/organisation objectives and you gaining expererance and skills that help you. Working as a dish cleaner is not that, in fact they should have a machine to clean the dishes as hand washing will not clean them to a standard that is now expected by the FSA. I digress, you should have a Volunteer agreement signed by yourself and the company/organisation, this agreement is like a employment contract in regards to what they expect of you, what you expect of them, your H&S guidelines, procedures for complants. who you report to and other matters that affect you.
If you wanted to continue you should ask that your role be defined and provide the above agreement. I personally would find it troublesome to hear that the aircraft where maintained by pilots rather than by trained maintenance personal.
If you where gettign paid to clean the dishes then i would agree with RTN11 but you are providing your time to help them, you time is worth something make them aware of that.
fats