Repairing Leaking Polyethylene Holding Tanks by Plastic Welding

lockman · December 12, 2008, 10:02 AM

Sent: 9/28/2004 3:27 AM

Owning old Winnies is such fun, we get to learn all kinds of new things. Like the questions we should have asked the PO, the ones he was not volunteering information on. Then how to fix all those things he knew, but we didn't even know could be broken. Our 78 D23 came with white polyethylene fresh and waste tanks. Not one of them was water-tite. Age and vibration had caused stress cracksiant engineer figured they could just suspend the waste tanks by there ends only, with bolts and angle iron. Might have looked great in 78, but not pretty now. Radial cracks at all the bolt mounting areas.

To be honest we did notice the repair attempts on the fresh tank when replacing the counter top and cupboards. We even filled the tank to the level of the patch and no leaks, Great we thought, the reality was to fill higher than the crack. The weight of the water (hydraulic force?) worked its way past the old repairs and all of the new attempts. And there was a lot of them, so don't waste your money on any of the following. The PO oringinal repair was white marine silicon over old dried up SealALL. Marine -Tex was next, made a nice hard patch that promptly popped of the smooth plastic. Plumbing store was next, if it was a solvent, glue or bonding agent we tried it, same results. Auto store next, lots of plastic repair products, radiator, body panels bonders, epoxies, etc., all tried, not much better results. Even went to the industrial supply for the rudely expensive Devcon plastic welding (epoxy) kit. Claimed to work on most all plastic, well after it failed the water test we had words about this tank and plastic was not even close!!! But if you think about it, this thermostat polyethylene was designed to resist anything, chemical or other?? that would end up in a waste tank. It was just doing its job and resisting anything chemical. So with the fear of complete banishment from the campgrounds for leaking black water goo on the ground, we stooped to the dreaded RV dealers for help. With $ signs dancing in their eyes, in unison, they replied tank removal and Plastic Welding (if possible) was the only answer. With prices starting at just plain rude to totally obscene, and the last dealer with voice quivering in anticipation of a quick kill, offering to squeeze us in next week if we commit right now. That would give us just enough time to arrange the second mortgage to pay his " I'll try to keep it under $1500.00 " bill.

So decisions were made! We did not have time or money to remove the tanks, we had better learn to plastic weld. There were actually two separate sealing issues involved. The waste tanks were a slop problem not a pressure problem. Using what we had learned in our many trails, we made effective semi- permanent patches for the waste tanks. By repeatedly dragging a serrated knife blade across the plastic then again at 90 degrees a cross hatch pattern replaced the smooth one. True silicon was okay, but the best bonding was achieved with black urethane adhesive/sealer used in the autobody field, to bond body panels to each other, and glass to metal. By aggressively working a thin coat of the adhesive in with a small stiff brush, then a heavier coat, and while still wet, a layer of left over fiberglass seam tape from the roof embedded in till saturated. A final heavy coat produced a strong flexible, well adhered black rubber DIAPER. We were at least now waste tank leak free and socially acceptable again. The above method of course failed totally on the fresh water tank. In fit of well justified rage my brother in law (restoration partner) attacked "THE TANK FROM HELL" with his 100 watt Weller soldering gun and strips of plastic form an old ice cream bucket lid as welding rod. To his credit it did reduce the stream to a manageable weep, but was easily separated when proper repairs were done later.

As "The Old Girl" was now mostly usable, I started my search on plastic welding right here with the old discussions on tank repairs and found a link for a motorcycle fender plastic welder and lots of info at http://www.urethanesupply.com/ There was no follow up if the member actually used it though. I found elsewhere a Canadian supplier, "Northwest Polymers Inc" in Saskatoon Sask. (no website) and for logistics I ordered it there. The " no air" welder appers to be similar to others, it is a pencil type soldering iron with a custom foot on the bottom. Mine is a 35 watt model with a oval shaped, flat bottomed, cast brass foot. There is a hollow tube for manual insertion of the plastic rods. The foot is 1/2 inch long by 1/4 inch wide approx.

The instructions were fairly simple, and if I could have pre warmed the area, or even had room to work it would have gone smoother. The cracked area was under the sink area at the back of the drawers. Very little room to move and tight angles. First lesson came quick, WEAR GLOVES, this iron and the melted plastic do not play well with bare skin.!!! The process was to melt the under laying plastic and while molten inject more by pushing the plastic rod through the tube. It was not the welding process I was thinking of, more like building up the worn down teeth of an excavator bucket, but with a glue gun. The outside temperature was my big enemy as it was rainy evenings only for my work time. The plastic had a tendency to harden quickly around the edges of any area worked on.

The hand push feed was cumbersome in such a small place, and my desire to repair/build up as large and thick an area as possible made the going very slow. The end finish is not very pretty to look at but DAMM it does not leak, and isn't likely too any time soon! The new plastic bonded incredible well and appears to be harder than the original. 600 miles of mountain roads as a test run prove that. The urethane supply site suggests embedding a stainless steel mesh for some repairs, and I will most likely use that for the permanent waste tank repairs next spring. With only a month of season left, I will continue to use my rubber diapers for now.

Sea Hag · July 16, 2012, 12:25 AM

Sent: 9/28/2004 12:04 PM

I have a similar situation on my 76 d23 but only on the waste tanks . all of the bonding agents don't stick to the poly for long ( most products state in very small print Not for poly ). I did manage to make some marine epoxy and fiberglass mesh for reinforcement work on the black water tank . held up for a year now still has a very slow drip . Mine are both cracked by the outlets due to non support of the dump manifold . I plan to remove them and attempt a plastic weld . But for $1500.00 WOW were they going to line them with gold ? I have seen some new tanks that look to be the same size and dump outlet locations . mountings for the top angle iron mount for $150.00 to $200.00 dollars . I think they were at RV Surplus salvage . I believe you can still get them from Winnebago but probable more expensive - Winnebago made a lot of the plastic extrusions or had an exclusive manufacturer for stuff like the tanks and vent kids . Sea Hag

Sea Hag · July 16, 2012, 12:27 AM

Sent: 9/29/2004 10:16 AM

Lockman , thanks for the link on the welder . Ive been looking for one . at $35.00 you can't go wrong . You should post this link on the parts board .
I used a similar welder years ago on poly containment sumps . it was much bigger but poly was much thicker . they work well once you get the hang of it . It's a good idea to find something similar to practice on . . The winnebago RV service manual recommends plastic welding . One tip It says to drill a hole at the end of a crack before welding it to stop the crack from continuing . Sea Hag

DaveVA78Chieftain · July 16, 2012, 12:29 AM

Sent: 9/29/2004 3:40 PM

Lockman,
I suspect your reference to another member who led you to Urethane Supply Co. may have been a post from me a while back. No, I have not ordered and used the welding kit yet but, I do plan to sometime in the future. I had radial cracks radiating out from the mounting bolt holes on the fresh water tank. They occur do to stress from several hundred pounds of liquid moving back and forth (sloshing) when the rig is in motion. Next time you see a farm truck hauling a plastic tank of liquid fertilizer watch just how much that liquid moves and gyrates in the tank. Now think 8 lbs per gallon and thats a whole lot of weight being thrown around. For my 65 gal fresh water tank that equates to around 500 lbs of liquid motion when full. I was able to successfully seal my fresh water tank for now using the plastic bondo style tank repair kits. Trick with those is that the plastic has to be sanded well, cleaned, and use several layers of epoxy. Drilling holes (stress relief) at the ends of the cracks is also mandatory or the crack will only keep walking outward. I view this as only a temporary fix though. Either plastic welding or replacing the tank is the only real solution. You can get the ATV repair kit at Urethane Supply Co. for $40. I would also recommend the stainless steel mesh too. The important thing will be to beef up the area that has become cracked do to mechanical stress. That's all you really need for this sort of job. The tanks are hidden so making the weld pretty is not a real objective in my mind. For those interested, they also post all of the directions on how to do this on their website. There is also a Haynes book available on how to do plastic repairs. It to recommends welding as the solution for plastic repairs. It also explains how to do many types of reconstruction as well as painting on plastics. It should also be noted that most new tanks today use spin welded fittings to attach the fittings to the tank. Myself, I am not so sure I am comfortable with the idea of using a router (yes a high speed wood router) to do that spin welding.

Even though I had already been sold on the welding approach, thanks for your feedback on your efforts so far.
Dave

Cooneytoones · July 16, 2012, 12:30 AM

Sent: 9/29/2004 10:54 PM

Another thing to fix when done welding or patching a cracked tank is, where the dump lines T's just before the valve. There is a PVC T that connects the gray and black water tank. Older plastics ( from the 60's and 70's and even early 80's) is not quite as durable, expandable, or strong as today's polymers. When they built these things up til the eary 80's there was no such thing as a no-hub coupling. (Rubber coupling with hose clamps on each side) It wasn't until 1985 or so that they were approved for use in most city plumbing codes. I believe the National Plumbing Code adopted them in 1985. The gray & black water tanks are connected with a tee and SOLID PVC pipe. Big mistake, since both tanks are independent of each other, the give and take from sloshing and road bumps wreak havoc on the connections from the tanks to the dump T (on either side of the dump valve) and from the tank hangers, this is some of the cause of stress fractures. So, if you replace tank, or fix old one, make sure you get two Rubber No-Hub Couplings, about 6 to 10 dollars each at any hardware store. Just cut two 1.5 to 2 inch pieces of PVC pipe and glue them into the T on both sides, and use the same length pieces coming out of each tank with the rubber couplings in between. This will solve a major problem, that will keep re-occurring if you make a solid connection (like the one it came with) between the two tanks.
Timmy

kd4pbs-1 · July 16, 2012, 12:33 AM

Sent: 5/22/2005 11:25 PM

Thanks for the info! While re-doing the carpet in our Itasca, I happened upon a large wet spot under the dinette where the water tank lives. I pulled it out totally, and have just ordered the "ATV fender repair kit" and some polyethylene welding rods from the urethane supply company. I was searching for a replacement tank, and noticed that there doesn't seem to be anywhere to get one. My only other choice was to get a smaller tank that would fit, but not fill up the whole space. I much prefer this option.

kd4pbs-1 · July 16, 2012, 12:35 AM

Sent: 6/1/2005 11:29 PM

Time for a follow-up...
After waiting two days for the kit to show up, (I ordered it next day air) I had to call urethanesupply.com and ask them where my order was. Donna answered the phone and told me that she couldn't find my order anywhere. After playing tag back & forth, they found the order and admitted they screwed up. They sent me the "ATV fender repair kit" and a bundle of clear polyethylene welding rods overnighted, and refunded me all and more of the shipping charges. They had excellent customer service, and I applaud them for doing the right thing.
As far as the quality of the repairs this kit makes, I'll have to say that it seems the tank is as good or better than new. It is very easy and straight forward to weld the polyethylene back together. I had lots of cracks (probably 15 or so large ones) and all of them filled up nicely. I filled the tank to the top, and there were no leaks.
Now, though, I found that the gray water tank has a large leak. Looks like I'll be putting this kit to more work.
Oh, it repairs ATV fenders quite nicely too

-Matt

Oldbag373861 · July 16, 2012, 12:37 AM

Sent: 6/2/2005 2:26 AM

Wow, all you guys are scaring me! So far i haven't seen any water-black or fresh- under either of my tanks. I don't know if I want to look! "If it ain't broke...".

DaveVA78Chieftain · July 16, 2012, 09:39 PM

Update
As I indicated the plastic bondo style tank repair kit was intended to only be a temporary repair. Sure enough it starting leaking again last year. So, I did the plastic weld approach. I had several radial cracks that developed from vibration and water weight shifting around the hold down bolt holes on the freshwater tank. I also had age related stress cracks around the black tank output fitting. A PO had also tried to fix road damage to the blank tank with fiberglass. So, I had much to get fixed up. I first purchased the Hrbor freight setup. Did not take long to discover the flat iron tip that screwed into the iron wa very weak and barely lasted for one repair. So, having a 80 watt iron for stained glass with a 1/4 chisel tip I moved to using that which worked fine. While you can purchase polyethelene welding rods, I simply went to the local hardware store and bought a 15' roll of polyethelene tubing used to connect water up to a fridge icemaker. Worked great.
Make sure there is no water in the tank in the area to be repaired. The heat from the iron will boil the water resulting in steam seeping (expanding) through the solder repair ending up with an unsealable leak. A real pain. No water in repair area is a must.

Dave