In the past I have used something I believe was called 'Conseal TU' and was made for painting shipping containers - so it was pretty tough. The key thing is your preparation, get rid of all the crap first and use a zinc based primer in order for it to stand a fighting chance. You may also consider lifting the tub so you can do the whole crossmember rather than just the bits you can see... from experience, it's the bits you can't see that rust and then cause you the problms.