i filled it right to the top and i noticed fuel was dripping from the small screened hole in the centre of the cap.
No, it's not the amount of fuel. The tank on a Weed Eater XT300 trimmer is inverted and so the cap is designed to beÂ below the gas level all the time. I think you have a problem with the duckbill venting valve in the cap. You can take the cap off and take a look at the valve in case there's something in there that prevents it from closing completely. Or simply get a new Fuel Cap Assembly - Weed Eater Part numberÂ 530014347. It's not a very expensive item (about $4) but can save you from some third degree burns which may result from the leaking gasoline getting onto the hot engine and catching fire if you keep operating it like that. Be safe!
Stop for a small bit and check if the rust is indeed coming out and resume the shaking process. Empty the acetone out into a sludge bucket and repeat the process.