If its a steel frame strip the existing paint job with a soft wire brush, clean up welds with a stiffer one, rub down in coarse paper, dry, spray with brine and shake on table salt to parts you want more rusted, wait a few months, wash and then rub down with a wet and dry paper, wash down, dry it perfectly and keep it by a radiator for a few days and use a matt clear coat.
Hey presto rusted frame thats now more or less weather proof and ratty.
If its a steel frame strip the existing paint job with a soft wire brush, clean up welds with a stiffer one, rub down in coarse paper, dry, spray with brine and shake on table salt to parts you want more rusted, wait a few months, wash and then rub down with a wet and dry paper, wash down, dry it perfectly and keep it by a radiator for a few days and use a matt clear coat.
Hey presto rusted frame thats now more or less weather proof and ratty.