Saturday, July 21, 2012

Part 22 - Sealing the gas tank

This is a highly contentious topic. The chopper forums are riddled with strongly polarized opinions. Bottom line is that your custom gas tank has welded seams, mounting points, petcock bungs, crossover bungs etc that could all potentially leak. Leakage is caused by microscopic pinholes that occur during the welding process - these allow a tiny amount of gasoline vapor leakage to the atmosphere. Over time this causes the paint to bubble destroying your paint job. If you paid for a custom paint job, you are easily anywhere from $2000 to $5000 in the hole so you decide whether you are prepared to risk it. A properly welded tank should not leak.

I welded crossover bungs into my tank. While it appears to be fluidically tight, I can't tell if there are pinholes. Pressure testing is difficult to do and there is no telling whether stresses from vibration won't open up a pinhole leak point. The other thing to consider is rust prevention. The conventional wisdom is to keep gas in the tank - the vapor displaces the oxygen which prevents rust. Having seen rust develop inside my tank because the gas cap leaked during wet sanding, I felt more comfortable sealing. Much research on the web yielded 4 candidates

  • Kreem - most opinions recommend to avoid this product. Hard to tell if this is because of sloppy application or what. I find it hard to believe that a poor product would be on the market as long as it has been. 
  • POR-15 - what I used. A lot of people swear by it but there are some negative opinions out there. As with all products, following instructions is key to successful application. Has well proven rust prevention characteristics.
  • Redkote - similar opinions to POR-15.
  • Caswell Epoxy - similar opinions to POR-15

You can Google each of these products and make up your own mind. 

If you are going to seal the tank, you should seal before you paint the outside of the tank!!!

The first two steps in preparing the tank for POR-15 are to clean and etch the insides. First an application of Marine Clean dissolves all grease and oils on the inside of tank. You can seal the ports in your tank with Duct tape. Follow the instructions - 

Next, treat the tank with Metal-Ready. Follow the instructions. The jug on the left has the dirty Marine-Clean that came out of the tank after 2 hours. Each surface needs about 20 minutes of cleaning time. I did a lot of shaking to agitate the cleaner periodically. 

Each surface within the tank needs to be exposed to the Metal-Ready for about 20 minutes so you will need to rotate the tank into a new position for each exposure - same process as for the Marine-Clean. The Duct tape did leak a bit while the tank was upside down but this was minor. After Metal-Ready is complete, the tank needs to be emptied and rinsed with hot water a number of times. They recommend no longer than 2 hours of exposure to Metal-Ready. 

All in all, roughly 4 hours of cleaning time..... fun hey!

The next step is to thoroughly dry the tank. It must be absolutely dry. Use a hair dryer or heatgun to force warm air into the tank to dry the inside. I also baked the tank in an oven at 200F for a couple of hours after this to make sure that it was absolutely dry.


Then the POR-15 gets mixed thoroughly and poured into the tank. Make sure you have all holes plugged properly. This stuff is messy and sticks like crazy to anything it touches. Lacquer Thinner will clean up small "accidents". Keep rotating the tank to make sure all surfaces get properly coated. 

My tank has a vent line. The sealer plugged the vent line so I had to keep blowing air through it to keep the pathway open. When you are sure you have everything coated, you can open one of your plugged ports and drain the excess sealer back into the can and leave it to harden for disposal. The most important thing to remember is to read the instructions!

The sealer needs at least 4 days to cure before exposure to gas. Play it safe and add a couple of days....


  1. I've use Kreem and POR15. Both work.

  2. Sediments and other impurities formed from the natural process of fuel degradation can also gather and settle in the bottom of the tank.