It can be done with the right preparation. Instead of sanding try using a medium grade scotch brite pad and you'll find it easier as it won't clog up like wet & dry paper does and you'll be able to access more areas. No dust that way either. The idea is to take the sheen off and give the paint something to adhere too. This is what panel shops do when repairing powdercoated bull bars etc. You'll get the best results using a 2k paint. Powdercoating can be very difficult to remove. Also the powdercoater could put another coat of the correct colour on the frame to fix your problem.