A while back I purchased a dent-free, yet crud filled gas tank to replace the one on my '89.  I purchased the KBS motorcycle restoration kit, and went to town on the tank last weekend.  In another couple of weeks, I will have to repeat the process because I followed the directions on the products.  Here is a list of things I've learned the hard way:

-If your tank is full of garbage yet there aren't any holes in it, let it soak overnight.  This is not listed on the directions, but the guy at their 800 phone number suggested this.

-Jump online and buy a long, narrow, bendable wire brush if your tank has lots of scales in its interior.  You'll want to scrub the inside of the tank after you've drained the acid from the tank and are flushing it with water.  Lots of junk will brush off.

-Buy a THICK length of chain to put inside the tank when using the acid step of POR15 or KBS.  I used a 16 gauge chain thinking that smaller was better, and it caused two problems.  First, the chain didn't knock much rust loose.  Second, the chain got stuck in the crud pocket and was very difficult to remove.  Again, go with a chain that is thick, but still light enough to be removed with magnetic tools.

-There are crud pockets on the left and right side of the tank.  These pockets can be seen with a flashlight.  Get used to hating them.  They take forever to dry, and hold all of the junk you are trying to remove from the tank.

-If your tank is as full of junk as mine, you'll have problems getting all of the sediment to drain out of the hole that the petcock normally resides in.  Consider buying a pump of some sort to suck out the garbage that won't nicely flow out of the little petcock hole.  I'm going with the BiOrb hand pump from PetSmart. 

Expect the first two steps of the Por15 or KBS system to take an entire day.

Oh, and everything will cost you about $50+the tank+painting costs.  It may be cheaper to just buy a pristine tank at a higher price.

Thanks for the info, may I suggest that this makes it's way to the wiki?

Rylon

The first tank I did was with the whole por-15 kit, and it went pretty well. I think I used simple green instead of the marine clean though.

The last one I did, I still used simple green that I already had, but then I just bought some phosphoric acid($16) from home depot, and then bought the tiny $11 can of por-15, so my total was around $27, and it still worked perfectly fine.

The only thing I wasn't sure about, was how much of the phosphoric acid to rinse out of the tank. POR 15 instructions have you rinse the tank before you coat it, but most places that talk about the acid said that if you leave it, it will leave a thin coat in your tank, and keep it from rusting again within hours before you coat it. I left it, and it worked fine, although it didn't dry smooth. Once I coated it with the por 15, and looks kind of bad, but I know that's just the coating the acid left. If I do it again someday, it will be simple green, phosphoric acid, a couple rinses with water, maybe alcohol to help it dry, and then just the $11 can of por 15 again.

Thanks for the info!

Phosphoric acid is a great one for treating rust, just that I dont use it in a tank. There is too much dust it makes and getting it all out with out re introducing water is a huge PITA.
Chain and this and that is all a big waste of effort IMHO.

Full strength muriatic acid, do it in stages if you wish, is the best first step.
You can once you get a rust free tank, put the POR15 acid in it (which I suspect is a cousin of phosphoric acid cos it also makes grey dust, just a lot less of it) and 1-2 hours later rinse off the tank, then dry it with towel or hot air or somehting. Then it can sit for a few days while it dries out 100% in a closed car in the sun. Then coat with POR15. Or run a little acetone through it and coat it.

Anyway these are all over the wiki and in the FAQ's etc. No sense saying any of it again.

Anyway I'll POR15 a tank for 100 and reverse shipping if its not got a prior coating.

Cool.
Buddha..

Buddha,

In tanks that contain lots of junk, more than just acid is needed.  For the tank that I recently purchased, I followed the instructions from KBS, specifically regarding the acid step of the tank prep.  By following the instructions and multiplying the tank soak time by 2, I has still left with lots of garbage in the tank.  The problem I was running into was that I would try to flush the tank yet junk would still remain inside of it.  I needed something to agitate the garbage in the tank, and a method of emptying the tank that drained out the stuff that refused to come out of the tiny drain hole. 

Perhaps the chain+wire brush aspect is excessive, but at the very least a good brush with a long, bendable wire handle is helpful, along with a water pump.  The total cost of these extra items runs from $15-$20, which I believe to be cheap considering the extra insurance they provide in the case of badly maintained tanks. 

This is just my experience, YMMV.

Ha ha ... I punch a hole in the front top side of the tank and put a drain in it. That lets all the crap come out with hot water.
Then acid.
Of course you can do it all with acid, just have to find a way to drain it all out quickly and safely. That stock petcock hole is neither quick nor safe. Any tank that is beyond surface rust, gets a hole ...

Cool.
Buddha.

I am putting my bike away for the winter, and I need to empty the tank, so I can take it to the body shop to fix a dent it had when I bought the bike.  I took the tank off, and drain as much gas off as I can from the valve at the bottom, I even turn the tank upside down to pour gas into the funnel, but there is still some gas left in it.  If I take the valve at the bottom of the tank off, would it empty the tank?  Thanks,