I figured I'd give my 2 peneth in my humble opinion and share my research for those who are starting a UDS build and have halted at this first hurdle.
The red/orange/tan "epoxy phenolic resin" liner is almost impossible to burn out and takes considerable time and effort to wirewheel out. You'll put your lungs and throat at more risk of damage trying to sand it out without using the CORRECT breathing equipment with fit for purpose filters, not just some old dust mask from your tool box.
This stuff is NOT just paint regardless of any over spray marks at the rim you may see, it's just the way they applied the resin before baking in the manfacturing process.
I did spend a lot of time trying to work out what this stuff is and if it was toxic to humans!!
I even Contacted a couple of paint/epoxy resin manafacturers and waded thorugh a lot of technical material data spec sheets and replies from them.
General overall consensus is that it's harmless! Anything that might be harmfull will be burned out in the initial hi-temp 'burn out' you do to burn the outside paint off.
This stuff could be used to succesfully coat the surface of the sun! Your never going to cook or heat it up to over 1000F which is what you'd need to do to start breaking it down.
So if you can't get all the liner off - seriously, I figured it's not a problem, just ignore it in my opinion!
You're never going to clean the inside of the drum, so after a while (very quickly in my experience, 1 or 2 cooks) it'll be covered in lovely black smokey oily residue anyway, sealing it all safely in.
http://www.heresite.com/products.php
Yes, Muratic acid will eat the red liner out. Seems more risky than cooking with it in though! My friend has got a top chemistry degree and can't see how leaving the liner in would hurt at all at the slow n low cooking temps of anything from 150F-400F at these temps the liner is barely luke warm in comparison.
My research certainly raised my comfort level a lot. With just a little reassurance from some science input.
i would certainly be careful eating the liner out with the Muratic acid as it lets out some really toxic/noxious fumes.and is probably more harmful/dangerous than anything else mentioned.
"Epoxy Phenolic barrel coatings feature improved flexibility and adhesion, and become (after baking) non-toxic, odorless and tasteless. Subsequently meet the requirements of the FDA Regulation, Title 21, CFR175.300 for direct food contact.
Other than that - it's been covered
I've heard of people giving up completely because they can't tackle or face the liner issue,
and other's saying yes you have to remove it and it's all part of the "fun" of building one and how you'll get more satisfaction at the end for doing it ?? errrrrrrr I beg to differ. 
I personally had 1 nice hot fire in my barrel for 3-4 hours until the sides of it starting glowing red in the dark, I wire wheeled over it to remove any loose bits etc then left it as it was, gave it a wipe out/wash and a clean then dipped a rag in veg cooking oil and wiped round the entire inside/bottom/lid underside twice to clean and season/coat it thoroughly. Sprayed the outside with 2 coats of BBQ black paint, job done & Good Times
After the first couple of times smoking/cooking the entire inside is now coated in lovely smokey residue and all the food has been superb. Good luck if your building a UDS and make sure you share the experience.
G
