I have used the Kreem and similar "white" sealants before.  They work OK......but I believe the POR15 product works better.

They have a kit:
http://www.por15.com/CYCLE-TANK-REPAIR-KIT/productinfo/CTRK/

But I believe it is more economical to just buy the sealer and use my own products to clean the tank:
http://www.por15.com/US-STANDARD-TANK-SEALER/productinfo/TSG/

When I buy the sealer only I first use Purple Power or a similar cleaner to get rid of any fuel/oil residue, then a rust dissolver or phosporic acid to remove the rust, then rinse with warm water, then rinse with acetone to remove all traces of water (rinse until the acetone comes out clear and not milky).  Then use the POR15 as instructed.

8OZ is enough to do most tanks - and once the can is opened it cannot be used again as the product cures with exposure to the air/moisture and begins to thicken even it returned to the can.  If you have multiple tanks to do you can pour from one tank to another and can get a pint or quart.

I looked into it for an older tank, and found that Por15 seems to be the most recommended one out there, but never ended up using it. The tank I had cleaned up better than I'd expected, and never needed to be sealed.

I was also told to take the tank to a local radiator shop; I guess those guys have a way of sealing radiators that works even better for fuel tanks? Again, I don't know... Anyone out there ever heard that before?
I was told the guys here could seal the tank for me for that same price or less than the Por15 kit... I dunno for sure....

The POR15 is not bothered by ethanol.  I have coated several tanks that did not leak or where not rusty - I just wanted to prevent them from getting rusty as a result of having ethanol from pump gas in the tank.

One of the things I don't like about the white colored sealers is the coating is very milky looking, and it is somewhat transucent.  It looks odd in the tank and draws attention to the fact that it has been sealed.  The POR15 is a dark silver color and looks pretty normal.

I have been able to spread out the cost of it by buying a quart and immediately upon opening the quart I dumped half of it into a pint jar and sealed it.  The rest of the POR15 was then used to seal 4 Honda MiniTrail tanks all at the same time.  You can dump the sealer from one tank into another after sloshing it around - you just can't store it once it is exposed to air.