I have recently bought a 1981 924 and the fuel within the tank has turned to varnish after 16 or so years of sitting there, my question is what's the easiest or most reliable way to remedy this? Remove the tank? I am not mechanically minded so I wouldn't know where to start. The fuel pump needs replacing so now is a good time to get onto it.

 

You can try flushing it with fuel in place... though if you're going for a restoration-quality job, then pull the tank (which will required dropping the transmission) and have in "slushed" - cleaned and treated/re-sealed.

 

Are you planning to develop mechanical skills or willing to spend thousands (many, many thousands) of dollars on deferred maintenance? I'm afraid after 16 dormant years, the tank and pump will likely be just the beginning of a long resurrection.
A place to start is in the cargo area, remove the fuel level sending unit from top of the tank. This will give indication as to the condition of the tank internals. If the tank is full, the varnish / fuel can be dealt with while tank is in the car. If the tank was not full, there will probably be rust above the fuel level which generally requires removal of the tank to remedy.

 

Awesome, thank you I will give that a go!