I've used straight vinegar a lot, and it works very well - see photo. Haven't tried adding lemon juice and salt yet. Vinegar immersion in half day increments is about right. If it's not good enough yet, give it another half day, and check again. Afterwards, a simple rinse in hot water with dish soap, followed by thorough hand wire brushing prevents flash rusting. That photo was taken January, 2011, and 15 months later, those parts still look exactly the same, awaiting phosphating.

I've also been experimenting with Evaporust, which is a heck of a lot more expensive than vinegar. It works a lot faster than vinegar - inspection every 1-2 hours is about right. Evaporust can't seem to quite get the job done right without some help - usually some elbow grease with sandpaper or wire brush is needed, which is okay with me. I think it is always best keep the exposure to the chemical solution (whatever it is) to the absolute minimum needed, not any more. Periodic removal from the Evaporust or vinegar and inspection to find stubborn places that need some minor physical rust removal with sandpaper, wire brush, or light scraping with a file will shorten the immersion time a lot.

If using a costly rust remover like Evaporust, it's best to remove all loose and easily removeable rust by hand before immersion. That way, you aren't wasting and depleting the strength of the chemical needlessly.