So these plain metal gas tanks come in a box filled with warning stickers and flyers saying "Don't put gas in this tank until you've test-mounted, pressure-tested, and lined it." I think it specifically said to use "Kreem". So I bought the full Kreem kit from Carter Honda.
It's pretty simple, a lot simpler than the instructions sound. It basically goes like this:
1. Rinse out the tank so there's only bare metal and water inside.
2. Etch the metal with Kreem A (mixed with warm water).
3. Rinse, then flush it with Kreem B to dry up the water.
4. Pour in the Kreem to coat the tank.

Step 3 and 4 must be done all in one sitting or you'll get flash rust and have to start over. Step 2 requires you let the tank stand for hours. So either let it etch overnight, or start the whole project in the morning to get through it in one day.

Before you start, you need a few things.
A. Make sure you can seal all the tank openings water-tight. Plastic or rubber is what you want; duct tape won't work and a metal plug will get etched along with the tank. The tank came with a gas cap for the top and a rubber boot for the bottom hole. The problem is that the gas cap is vented by a little hole.
So to seal it up I made a makeshift gasket out of a balloon, which I cut into a circle and sandwiched into the gas cap. To protect the bottom of the cap, I put another balloon around the whole thing.
The rubber boot on the bottom worked great as is. I tested with plain water; the tank held water right side up and upside down, so I was ready to go.
B. I needed a basin to hold the tank and catch any spillage. (especially if my seals failed at some point during the 4 hour wait.)
C. A container to drain the etch into when step 2 is done. The problem is you've added 2.5 gallons of water to the etch, so it's not going to fit back in the original bottle. I bought a plastic jerry-can to hold it.

Step 1. Since it's a brand new tank, I just had to rinse it with acetone to get any oil off the inside surface. Dump it in, seal both ends, swish it around. I flushed it with water a bunch and I was ready for step 2. (A little water left behind is fine, because the next step involves water)

Step 2. Pour the Kreem A etch in, then add 2.5 gallons of warm water. It eats into the metal to provide a nice surface for the Kreem to stick to. It turns shiny metal into a flat dull surface. It's designed to cut through rust and neutralize it for reconditioning old tanks. As the tank sits, you can hear it fizzing, plus the water seems to stay warm... Leave the top cap off so the pressure vents. Every surface needs 4 hours of contact, so since the tank holds more than 2.5 gallons, I had to let it stand on one side for 4 hours, then invert the whole thing and let stand for another 4 hours.

The long wait is over, and now you have to finish without stopping. Draining a gas tank that's got internal lumps and grooves that trap water is hard enough, but it's made harder when you dry to get every dribble to land in a jerry-can (even with a funnel). This is where I discovered that while plastic and rubber aren't affected by the etch, human skin is. Not a big deal, but gloves are an idea. Then rinse rinse rinse until there's only water inside. Drain it as best you can.

Step 3. Out into the alley for this one. This step will remove the water droplets left behind and prime the metal. Dump in the Kreem B, which is simply Methyl Ethyl Ketone. A well ventilated workspace is key. Seal both ends, and swish it around well. Drain it back out, and whatever is left behind will quickly evaporate. At this point the tank is as perfectly clean and bare as it'll ever get.

Step 4. Dump in that Kreem. Seal both ends and slowly rotate the tank in every direction to get all surfaces coated. When you've spent some good time doing that, set it down, open the top, and let the air cure the exposed areas for 8-10 minutes. Then seal it back up and do it again, this time rest it a different way to cure the areas it couldn't touch the first time. You can repeat again if you think you're too thin in a specific area. Drain off the Kreem that's left and bottle it back up.

Open both ends and let air pass through it to cure it. Don't blow anything high-pressure through it obviously. After 6 hours you can do another coat of kreem by repeating Step 4. After at least 24 hours you're supposed to be able to put gas in. I'm giving it a few more days than that.

Oh, when you're working with the Kreem, you should probably wear gloves. This stuff really refuses to come off your skin. And it won't come out of your jeans either.

email: mikebike@rocketcar.net