Hello Bill.....
Repairing cracks in "poly" plastics can be a real challenge. The first step is to make certain you know exactly what type of plastic the tank is made of. Then I would suggest you call 3M tech support for recommendations from them.
The black tank on my motorhome developed a small 1 inch stress crack. The tank is made of high density polyethylene. I quickly learning that nothing...and I mean NOTHING will stick to or glue HDPE. The only solution is plastic welding.
Plastic welding can be done relativity easy. There are 2 methods to use...hot air or welding iron. At a Harbor Freight store I was able to purchase a welding iron for $10 and it included a couple dozen poly rods. The rods are about the same size as a piece of spaghetti.
Next step was to practice plastic welding. I looked around the garge and discovered several items were made of HDPE. The letters HDPE are stamped on the item. I chose to practice on a plastic dustpan and found the process to be rather easy.
Now it was time to fix the black tank. In my case I was lucky the tank crack was assessable but I had to lay on my back and do so contortions. After an hour twisting my body every which way I was successful in repairing the crack. That was about 2 years ago.
Unfortunately now the gray tank has developed a small crack in the exact same location as the black tank. So sometime this week it is time to get out the $10 welding iron and do the fix.
Here is a link to Harbor Freight.I bought the 80 watt kit when it was on sale for $10. Best money I ever spent. Give it a try.
I should also mention I learned much of this information as a result of a long telephone conversation with the engineer at the company in Indiana that manufactures the black & gray tanks in my motorhome. That fella had a wealth of knowledge & experience. It was he that said "NOTHING will stick to HDPE". The stuff was developed for that reason.
Hope this info helps you.
