Take it off, rinse it out with Marine Clean or similar, then let it sit in a dry place for about a month. Come back and then measure it. The best place to measure is the width between the bolt holes of the rubber bumpers on the bottom. You can compare to a metal EFI tank. They should be within about 5mm (I've see differences up to about 5-7mm between METAL tanks, so I assume this is sufficient margin for a comparison to plastic tanks).
Here is my basic procedure:
You need:
1 Caswell's tank coating kit (either one, but the "Dragon Blood" stuff is easier to see)
About 4 oz of xylene
a roll of paper towels
a piece of rug or clean rubber mat to rest the tank on upside down.
square tupperware, 4x4x8" (8" tall to mix and pour caswells)
a glass measuring cup (for the xylene)
tiny detail brush, steel
#9 rubber stopper
#10 rubber stopper
home depot plexiglass piece, about 10x10
6mm washers
rtv silicone
exacto knife
wood stirrers (popsicle sticks or tongue depressors best)
small hair blow dryer
some kind of water based solvent like marine clean. I think the purple stuff is ok too, you will dilute it about 10:1 with water.
about 2 months of time to let the tank shrink and about 3-5 hours to do the coating properly.
1 box of nitrile gloves.
silicone basting brush (99c store has these by me...)
Once the tank has shrunk back to the original size, use a #9 rubber stopper to plug the INSIDE of the filler neck. Yes, the INSIDE. I can get my arm just far enough in and use my other hand on the outside to guide the plug in from the INSIDE and jam it in. You can do the same from the outside if you like, but you'll get the coating inside the filler neck some. PLUS if you do the plug from the inside, it gives you a visible gauge of how much coating you spread once the coating sets.
Make a plexiglass cover for the fuel pump hole. You can use a DVD or CD to approximate the bolt pattern diameter. A CD is bigger than the hole, but smaller than the size you need so measure carefully. When you've cut it so it fits and the holes line up, use some RTV silicone and put a THIN bead in a line just inside the bolt holes on your plexiglass to seal it. Use washers to spread the pressure. The thicker the plexiglass the better so it doesn't warp when you screw it on.
I put a hole in the plexiglass to pour stuff in and used another #10 stopper so I can plug it for agitating the tank. Use a 1 7/8" hole saw to cut a hole. You can use a 1 3/4" but you'll get more of the plug in with the larger one.
Now rinse it again with diluted marine clean, dry it well. If you've plugged it right, nothing will leak. This is a test of your plugs as the marine clean won't eat paint. Next, remove the plugs and use a small hair dryer on low heat setting. Dry the plugs too. The tank will be noticeably dry and warm. Unless you've got a huge arm, you can get your hand inside a good bit of the tank with a coarse metal brush or a "detail" brush. I used a steel one. Just rub it everywhere you can. This is labor intensive. Let the tank cool.
There is one hole in the filler neck that stuff can leak from even with the plug inside. You can use a 00 plug on that or just keep the paper towel in place with your hand. I've done both (we have a lot of stoppers around here).
Now find all the "pimples" in the inside surface of the tank that you can reach. They look like this:
You can either cut the surface off or just score it several times using an Exacto knife. The point is to open the pimple up so coating goes inside it. Don't dig. The tank is about 3/8" thick and the spots with bubbles are thinner. getting rid of these will make the coating adhere better and will seal the tank better. No random air pockets for condensation.
Now you've got a tank that you've rinsed, dried, shrunk, plugged, rinsed again, dried, cut and scuffed.. Still with me?
Put the cover and stoppers back in. You've verified they don't leak, right?
Put on your first pair of gloves.
Pour about 2-3 oz of Xylene into your glass measuring cup. Use the cup to measure it, about 10 feet away from the tank. Why? Xylene will eat your paint like a MOFO. Extreme caution in handling it.
With about 20 paper towels handy, carefully pour it in the hole in the plexiglass. Plug that hole hard.. Paper towel in the top filler? Now rotate the tank. Keep it up for a while. You are cutting the surface, diluting any last vestiges of gas and ethanol.
Check your gloves. Are they dry? Xylene eats paint.
Now put the tank on its top on your mat and remove the filler cover. Stuff about 10-20 paper towels into the filler hole to absorb the xylene. You will NEVER safely pour it out. Don't bother trying. Trust me. Just use the paper towels to absorb the xylene and remove them carefully.
Note: You've probably noticed I have not said "Cover your tank in plastic or tape." Why? Because if you spill xylene on that, it will not matter one bit. Xylene will penetrate tape and will run along your plastic and find an opening and then eat your paint. It is better to have none and be able to wipe it away FAST than have to tear off all the tape while it sits on the paint.
Now you've carefully removed the xylene paper towels. Put them far away. They are flammable and you'll get very high if you don't.
Gloves ok??
Now you can mix the caswell's. Use a large SQUARE tupperware. I use those disposable kind, they are about 4x4x8". Use wood stir sticks. Put in about 1 Oz of xylene to reduce and stir for about 10 minutes. No really, 10 minutes. Scrape the sides and bottom well and stir.
Put on a new pair of gloves. Just do it.
Dry the plexiglass and put it back on. Now you can pour in about half of the mixed caswells. With the xylene, it is MUCH thinner, so pour slowly.
With about half the caswell's in the tank, and your plugs confirmed in place. Turn on some music, get a comfy place to sit for about an hour and start turning the tank. For the first 5 minutes or so, just keep turning it in your hands. Every direction. Keep an eye on the top filler and make sure your plug + paper towel is holding. This is where the red caswell's is helpful. Caswell's WILL wipe off with a dry paper towel when you get it quickly. Just keep turning it about every 5 minutes now.
After about 30 minutes, open the bottom and move the tank around so the caswell's runs on the top and front of the tank. the point is to get the xylene to start evaporating and get some fresh air in side. agitate the tank with the bottom stopper out for about 10 minutes.
now pour in almost all the rest of the caswells. Leave about 2-3 ounces in your tupperware and put the top on it. if you have any xylene left, put about 3-4 drops into it before you cover it. Put this aside. Are your gloves clean?
now agitate the tank with the bottom open just move the stuff around. by now, you should see it clinging everywhere. put the stopper back in and continue agitating every 5 minutes or so. Turn the tank and then lay it on the bottom, top sides, front and back. The point is to get every angle covered and let it sit for a few minutes and then move on to the next angle. once the caswells gets noticeably thicker, then you need to speed up the rate of agitation.
the "problem" area is the spot right in front of the filler hole. there is a "shelf" there which is the bottom of the tank. the critical areas are the lowest points of the tank, i.e. the front and rear corners and the sides. This is were the condensations sinks and where the expansion starts.
you're going to be turning the tank at least another hour at this point. have a beer. open the bottom stopper about every 3-4th time you turn the tank to allow in fresh air and vent the xylene. windows open? gloves clean?
>hour passes<
Now the mixture should be crazy thick. Keep turning it until you don't see a "tide" of it rushing down when the tank is upside down. You'll know what I mean.
Once the "tide" is moving like honey, you're ready for the last step...
Now take out that small amount you set aside and the silicone basting brush. Give it a stir with one of your sticks. Keep stirring until it starts to get thick. It may take a bit of time, but you want it like honey.
You're going to stand the tank on its front end so the mixture goes to the front. Remove the plexiglass cover. Carefully. There will be drips, so use paper towels to catch.
Peer inside and you'll see the shelf and the hoses. Carefully use the basting brush to "paint" these with the leftover caswells. If you've let it get thick enough, it will cling just fine. You want to get as much of it on the shelf and the hoses as possible. It will spread by itself. Also, if you had any of those pimples near the opening, just gob them with the brush to make sure you push caswells into them.
Now that the plexiglass is off, you can either leave it off and carefully turn the tank OR you can clean it off and put it back on. If you do the latter, just wipe it with paper towels, it doesn't need to be perfect. Leave the stopper out and turn the tank.
Once you can't see it moving anymore, you will turn the tank for another few hours, but only once every 30 minutes or so. You're basically done. Once you've done this 4-5 times, set the tank on its bottom. Any drips out the bottom hole will go on your mat and who cares.
The coating takes about 3-4 hours to set and I wait about 3-4 days before putting in gas. Once it is dry to the touch (next day), remove the stopper from the top. It might have some stickiness around the contact point, so use your gloves and remove it carefully. You will also need to scrape a little from the outside lip of the filler hole. Use a plastic painter's knife.
Put your hair drying back into the tank on NO heat. keep the air circulating to speed up cure.
Keep the tupperware around. This will give you an idea how much the tank is set up. I've found the leftover mix in the tupperware still sticky on the plastic side up to TWO weeks later. Thats' why I prefer to wait after it's set before putting gas in.
This is how I do my tanks. My first tank was not done with this much OCD, but it is still holding strong and I did not use xylene to cut and prep the tank or reduce the mix (i used acetone). Acetone is ok, but Xylene is more compatible with nylon.
Hope this is useful.
