Repairing cracked black water tank

[DaHose]

From the day we bought good old Mabel, she's had a leaking black water tank. A PO somewhere along the line did a very poor job of repairing what I think is a crack at the valve fitting with some fiberglass. The tank is empty and today was about peeling off that nasty old repair and trying not to get spattered by the black water still dripping from the tank. GAHHHHH!!!!! :E That was highly unpleasant.

I didn't examine the tank closely to find the casting marks, but all the research I have done so far says that RV black water tanks are usually made from polyethylene. That means that I can buy some polyethylene tubing from Home Depot (the clear Watts tubing is made from LDPE) and plastic weld the tank this weekend. I hope it will be less unpleasant than I think it will. I will report how it goes repairing the poop tank. 😞

Jose

[DaHose]

Small update. I used the new turbo flush to put some water in the black tank and it still has a tiny leak. I didn't notice that the crack extended down in another direction. I will get that patched up and report back.

Jose

[DaHose]

Good progress on the black tank repair this weekend.

I cleaned things off, drilled the ends of the crack (it was all the way around the bottom), beveled the whole crack and welded the bottom shut. Then I welded in the reinforcing SS mesh and welded more material in over the top. I finished by welding material into the open crack so that it was just a little proud of the surrounding material. The plug I cut out of the tank when I put in the flushing system was actually enough material to do all the work. I can still buy some LDPE to cover the whole thing if I want, but the repair looks/feels solid. I also ordered a new blade valve and will install that before I try loading it up with water for leak testing.

I believe I also identified why it broke in the first place. Looks like a PO tried taking the tank out and a mounting bolt for a support strap is spinning (was probably welded in). I cannot get to the back of that bolt to hold it, so the locknut can't come off, or go further on. That leaves about 3-4 inches of play between the retaining strap and the tank. With that much movement, the tank can really sag when full. I think that the tank sagged, put strain on the outlet and cracked the bottom open. I have put in some rolled vinyl (minimal compression) to take up that slack for now. I will have to remove the fiberglass side skirt in order to have proper access to try and rig up an alternative mounting for the support strap, though. On the plus side, the outlet tube looks like it is well sealed and I won't have to dismantle/clean/reseal that part.

I will update after I install he replacement valve and leak test.

Jose

[DaHose]

Thanks for the recco. Art. I'll see if I can hunt down the supplies locally.

Jose

[Rvpapa]

DaHose,
Been there and done that. Remove clamp, heat up joint with heat gun and pipe will twist right out. To install clean inner and outer real good and smear both surfaces with urethane caulking, don't use glue. Caulking will stay flexible, glue won't. Give a quick spray with water (to cure urethane) and twist together. For clamps I used a couple of T-BAR (like on a turbo boost hose.
Mine has been good so far for ten years FT.
Art.

[DaHose]

Ok, I have some good information that I learned today.

First off, I now know that my tank is an El Monte Plastics, H-427w tank. That piece of information proved WORTHLESS however, as El Monte plastics is out of business and I cannot find information about my tank anywhere.

On the up side, I stumbled across the Plasti-Mend web site and found some really good information.

Plasti-Mend RV tank information page

From the Plasti-Mend Web Site wrote:
1.) For RV holding tanks, one of the quickest and easiest ways to see what plastic you have is to look at the pipe connection coming out of the tank. If the pipe is glued into the tank fitting, the tank will be ABS and Plasti-Mend will work fine for any repairs.

2.) If the pipe is clamped or screwed into the tank, the tank will be Polyethylene and Plasti-Mend will not work on it. The only reliable repair for the PE will be thermal welding.

Now then. The outlet is definitely welded to the tank, but the ABS pipes are not welded or glued. They are clamped to the outlet.




Looking at the plug I took out of the tank, that does not look like ABS. It looks like polyethylene.



I will still solvent test the plug, but if it is polyethylene then the rule is that you can weld HDPE with LDPE, but NOT the other way around. So my plan to use LDPE tubing from HD as my welding rod should be good to go.

Jose

[Bobbo]

Eternabond just adheres over the crack. The ABS/acetone mixture BECOMES part of the tank.

[gatorcq]

eternabond tape does a good job also. And it is easier to place over the repair job

[RayJayco]

Pirate wrote:

RayJayco wrote:

Teeton wrote:
If you do find it's a crack and you can drill small 1/8 inch hole at the ends of the crack do so.. That helps keep it from cracking... Just my 2 cents.

Yes, this is good info as it usually stops the cracks.
Most black tanks are ABS. Just put some ABS scraps in a mason jar and add some acetone (any of the 'tones actually) and let it sit for a couple of days. It will become a paste that you can cover crack and be the same strength that it originally was...

When does the acetone stop working? Will the paste continue to dissolve the tank?

No, the acetone evaporates leaving just the ABS when you open the jar and apply it. (You put the acetone and the ABS in a mason jar and screw the lid on tight until the ABS softens enough to spread in/over crack.)

[Chris_Bryant]

There is a lot of info on the Urethane Supply Co. website about identifying plastics (and repairing).

[Bill_Diana]

DaHose.....

Please click here to view a link that may be helpful & informative.

You need to confirm if your tank is ABS or HDPE. If High Density Polyethelene then you can plastic weld the crack. No glue will repair HDPE.

Click here for some info on how to repair ABS.

I might suggest your look on YouTube for more info on how to repair HDPE and ABS. The you might want to find something made of that material and do a practice repair before attempting on the RV tank.

Hope this info helps and keep us posted as your complete your project.

[DaHose]

The acetone flashes off eventually and the ABS sets back up. It's very convenient that pretty much all ABS is the same thing except for color and maybe some UV inhibitors.

ABS, pvc, polyethylene and polypropylene are GREAT plastics for making "stuff" because they are so easy to repair with some heat and filler rod as well as certain solvents. You end up with a like new repair with thermoplastics. You just need to understand what you are working with and have the right materials.

Jose

[Pirate1]

RayJayco wrote:

Teeton wrote:
If you do find it's a crack and you can drill small 1/8 inch hole at the ends of the crack do so.. That helps keep it from cracking... Just my 2 cents.

Yes, this is good info as it usually stops the cracks.
Most black tanks are ABS. Just put some ABS scraps in a mason jar and add some acetone (any of the 'tones actually) and let it sit for a couple of days. It will become a paste that you can cover crack and be the same strength that it originally was...

When does the acetone stop working? Will the paste continue to dissolve the tank?

[darsben]

Replace the tank. This is a job you only want to do one time.

Foolish savings in my opinion

[DaHose]

Put the left rear on blocks. YES! Thanks for pointing out the obvious, popeyemth. That will be a great help in staying yuckiness free.

The best fix for thermo-molded plastics is plastic welding with internal reinforcement using stainless steel mesh. I would definitely drill the ends of a crack. My process will be to clean, bevel/drill the crack, sand surrounding area and pre-treat with abs cement. Then heat weld. If it's not ABS, then I'll skip the pre-treat and go straight to welding.

I think step 1 is to be absolutely sure what the tank is made out of. I have plenty of ABS welding rod and I can always go pick up the LDPE tubing from Home Depot. You all reminded me that I have a core I just removed from the tank when I installed the flushing kit. If I can't find casting marks on the tank, I will use some ABS cement on that core and see if it dissolves.

Thanks for the helpful hints folks.

Jose