paint it by hand is the answer - use a roller for a good result ( i found a radiator roller better for the roof ) , and do 1 panel at a time
Paint I used was a general purpose metal gloss from Wick's (approx £5 ) and it seems to be ok ( though you are restricted in the colour choice )
here's a pic of mine half painted
its a shed - but at least its now a shiny shed