Qualcast Concorde mower - very blunt blades?

i have an old electric Qualcast Concorde mower that lately has been useless because it doesnt cut the gr properly and i think its due to rusty blunt blades. my main concern is how do i sharpen the blades and what with? i have tried using pieces of gl paper, sand paper, and a metal filer but no results, although the filer got alot of the rusty bits off. i would have gone and bought a new model however i have found from review sites that their newer models are apparently 'cheaply made' and are not very sturdy, trustworthy etc. thanks