Clean bike with nail polish remover
If the bike is extremely dirty and no normal cleaner helps anymore, you can just take nail polish remover for the steel parts! The cost only about 0.40 euros and with cotton very easy to perform.
Be careful that plastic parts do not come in contact with the nail polish remover as plastic dissolves! Likewise, rubber parts should not be cleaned with it. The bike looks like new again, if you have removed the rust with rust remover!