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

you gotta find the problem FIRST. before you can make a quality repair. or your just redoing what the first guy did.

The vast majority of black tanks are vacuum formed ABS, though there are some polyethylene tanks out there, that material is usually used in above floor fresh water tanks.

A good repair for the ABS black tank, is to start with ABS cleaner. After all is clean, and dry, put a layer of black ABS cement on the tank, quickly followed with a fiberglass screen patch, pressing into the ABS cement.After just a short dry time, add another coat of black ABS cement, to cover the screen. It is best to cover with several thin coats of cement, with a dry time between.Makes a strong repair, that is hardly noticeable.Jerry

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.

Put blocks under the wheels on that side to raise the valve above level so it won't drip on you.

Repairing cracked black water tank | Good Sam Community

DaHose
Explorer
Mar 16, 2015
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
Explorer
Apr 06, 2015
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

Forum Discussion

Repairing cracked black water tank

About Technical Issues

Related Content

Black tank repair?

Black Tank Crack

Gelcoat Cracks

Crack spreading

Repair of small Fiber glass crack?