Wow you guys need to relax. And to be fair, BNTFAM's comment didn't come off as D-Bagish , a little anal yes, but not aggressive.
OP use it as you would like, just be careful with these types of chemicals.
I'm a wrought iron fabricator, I've spent a lot of time dealing with anti rust compounds.. Fact is you can fight all day forever, rust will win, you can't beat nature.
The chem your talking about is a light acid that dissolves surface rust , but it also dissolves the steel (etch it), a small amount i'll agree, but it is still dissolving.
The biggest issue will be to neutralize it after your done, Steel is porous and the chemical will infiltrate into the steel so, any amount left in the tank will continue to work at slowly eating the tank.
Baking soda will work to a degree. Once you've finished with the acid, i'd rinse it with distilled water and baking soda, then dry it. Hair dryer or blow gun, as long as air is moving you'll be fine, do it in the sun on a warm day and throw a new rags in for good measure.
I'm against 100% using the liner, it just coats the rust, what you end up with is a poorly formed bladder around a rusting tank. I had one on my nighthawk the last owner did, the bottom of the tank was completely gone and you could see the pink liner, also they didn't empty the remaining liquid so it pooled at the bottom of the tank and took up about 3/4 liter of volume.. I had to cut the tank open and set it on fire to clean the garbage out, then rebuilt it.