You are looking at wiping down all surfaces, including walls and ceilings, with chemicals, running an air purifier or ionizer, replacing all carpeting, and that still may not take care of it.Hopefully their security deposit was a big one and you can get them out of there soon.

There are many commercial chemicals that can help, such as CLR.  

In addition to being from a farming family, I am also working on a PhD in chemistry, so I know a little something about the science behind this, and it is one of the most inexpensive chemicals produced today and costs very little to purchase.

While you can always come up with deviations to the past-performance-analytics ("What if the company's CFO runs away with all the money", "EPA fines for chemical dumping" etc on par with "Sinkhole" or "Need a new roof" ).  

I just got into my career as a chemical dependency counselor.

I owe that to you guys and "The Ultimate Beginner's Guide to Real Estate Investing" book.My wife and I are both process engineers (chemical engineers), so our finances are okay.

All the usual chemical treatments tried were useless.I have no idea what they put down there but it was the consistency of bunker C oil.The basement was finished in the area under the sink & tearing the old pipes out from below would have caused way too much damage/cost.Finally I cleaned it out with a small $99 power washer I got from Home Depot to do other things.

You can use a chemical de-glosser instead of sanding.Some varnishes back then had lead in them too, not just the paint. http://www.diylife.com/2010/03/08/how-to-degloss-a-surface-for-paint-prep/You need to use a contractor that is RRP certified.

I have found chemical treatment effective.

And I'll have you know - @Joshua Dorkin is much harsher on Detroit that lil ol me!