LERN ME: Plastic "Welding"

Some of you have seen my build thread on my newest acquisition: a 1979 Dodge W150 4x4 Power Wagon. One thing I've been fighting since I bought it in early January is a fuel leak in it's plastic gas tank. I thought it was a loose hose that had no clamp on it, but after fixing that, it's still leaking. I took it to the gas station recently, and it looks almost like it has cracks in the seams where gas just pours out of. That's not great.

Since there are no aftermarket fuel tanks available outside of expensive fuel cells or auxiliary tanks that take up a chunk of bed space, I've resigned to dropping the tank and attempting to fix it. After consulting some Mopar Truck-specific groups, this is the way to go. I've read up on plastic welding a bit, and I know that the fuel tank is made of a specific plastic that's resistant to chemicals, so it's not as easy as just slapping some goop on there and calling it good. I've read that a soldering iron and the correct plastic material melted into the crack will seal it up as good as new.

Currently at my disposal are a few soldering devices:
-A cheap, scary soldering iron with a flat bar end that I bought to fix the SID on my old Saab 9-5 (RIP)

-A regular-tipped Weller iron I use for electronics repairs (probably too small for this)

-A large Weller soldering gun (with a spoon tip!)

-A butane mini-torch with a soldering tip (probably not a good idea)

Can I use any of those, or should I splurge and pick up a purpose-built plastic welder? I have heard that people use soldering irons for this, but I want to do this once and the right way.

As I understand it, I need to drop the tank, de-gross it, drain the fuel, clean it out with a solvent like Acetone, and go to town. I plan on using air and soapy water to check for leaks afterwards. Seems pretty straightforward. 

Am I missing anything here?

In reply to Tony Sestito :

You are very much on track. The spoon tip Weller is your first line of attack. No butane for sure, we loose enough members to natural causes.

Any chance of tracking down a donor tank from the same model that you can harvest material from to use as 'filler rod'?  That way you could be sure the correct material is used.

Harbor Freight sells a couple plastic welder kits, but I don't know if they would work any better than the soldering irons you already have.  https://www.harborfreight.com/welding/plastic-welders.html

Though I doubt you haven't checked are you SURE an earlier truck metal tank won't fit? Seems like they made that body style for a long time. Or were they all plastic tanks by then?

Off the top of my head, the best plastic welders use hot air, not a soldering iron. 

I've done it before on a broken kayak using a propane torch and a butter knife. Heat the knife, not the plastic, and smear that E36 M3 in. 

Just make sure you use the right polymer type if you add filler material. In my case, the kayak was high-density polyethylene (HDPE) which has a recycling code of "2", which happens to be Lowes bucket material. So I cut the rim off a bucket and filled in the crack. Leak-free after that.

DIY Kayak Plastic Welding Repair

In reply to stuart in mn :

I had a hard enough time tracking down the truck itself. I haven't seen one around here in 15-20 years! There are other plastics that I may have kicking around that should work

I was looking at this plastic welder: https://www.harborfreight.com/welding/plastic-welders/plastic-welding-kit-with-air-motor-and-temperature-adjustment-96712.html

Not sure if it's necessary, but I want to make the process as easy as possible.

I am 90% certain the dodge tank is HDPE, and even more certain that it does not have a recycle number on it, cause too old. I used Scalextric track on a tank from an Alpine A110 successfully with the Weller "spoon" tip. again, too old to have recycle numbers, mid 1960s for tank and filler material.

In reply to ddavidv :

Earlier trucks used in-cab steel tanks. When they started using saddle tanks, Mopar used plastic ones while the others were still using steel. Pretty forward thinking for a company I always found to be behind the times compared to the other Big 3 manufacturers, but they probably weren't thinking that the plastic could become brittle 40 years down the road! 

I've seen people use behind-the-axle tanks for other vehicles. My friend's 1972 D100 is running a later Ramcharger tank, but he had to cut a hole in the bed floor for the filler. Most other steel saddle tanks have the wrong spot for the filler tube. Then, there's the problem of what straps to use, what sender to use, etc...

That sucks.  I'd be concerned about the general serviceability of 40+ year old plastic. 

If you go the "welding" route, be sure whatever you're using isn't hot enough to ignite gasoline vapors contained within the tank.

In reply to 1988RedT2 :

Yeah, it does suck. I plan on draining and cleaning it out with acetone and probably letting it air out a bit before blowing myself up.

Tony Sestito said:

In reply to 1988RedT2 :

Yeah, it does suck. I plan on draining and cleaning it out with acetone and probably letting it air out a bit before blowing myself up.

I think your main friend here is ventilation. If I were doing it, I would probably skip the acetone, concentrate on keeping the concentration of gasoline vapor in the tank below the Lower Explosive Limit. I would most likely do that by blowing tons of air through the tank for a while before welding, and during the process. Another method would be to put nitrogen/argon/carbon dioxide in the tank prior to/while welding. I would also have a fan blowing over the area to make sure vapors couldn't gather outside the tank at a level that could explode, because the spark from the switch in the soldering iron could be an ignition source. 

Why not just fill the tank with water from the hose before “welding” to cut down on explosion risk?  I think the main risk is from gas vapors igniting, but the water would displace those if you fill the tank all the way up.

You might consider bumper repair epoxy that you can buy at NAPA.

It's 2 part and I have used it for repairs other than bumpers.