My cylindirical cooler mash tun has developed two cracks on the inside and I melted some of the ouside. The result, I have a leak. Any thoughts about the best way to fix this? Should I use water weld by J-B weld?
The specs for Waterweld look like it will hold up to the environment you will be using it in. My concern is that HDPE does not always take well to adhesives and because of this the differing coefficients of thermal expansion between the HDPE and Waterweld may cause it to start leaking again after you have it repaired. If the stuff is fairly inexpensive, it is worth a try.
Food grade? Probably not. If you just patch the outside the interior bulkhead will get wort trapped behind it. Damaged equipment. Time to pony up for a new tun. It’s allright. I once burnt through the bottom of a cooler with a heatstick.
This gives you the chance to get a rectangular cooler! 
It does say it is approved for potable water and resistant to 300 degrees - though that does not mean it will make good beer. It just won’t kill you.
Throw it away and get a new one.
I bought J-B weld and a 50-quart rectangular cooler with a drain port below the bottom of the floor. I hope to patch it from the inside and get one more batch from the cylindrical cooler while I get the 50-quart set up. The cylindrical cooler may get some use for controlling fermentation temps in the winter.
Man I’d avoid that JB weld. It would be better to wait and brew on your new cooler (hopefully you bought a blue one). Probably only take a few minutes to convert your new mash tun over. You might be able to just slip the tubing right through the drain itself. I do that with mine and it fit’s just snug enough to not leak. Then I attach a ball-valve to the tubing outlet and the braid to the inlet with a small hose clamp.
and…make sure it’s Blue 
I have joined the cult of the blue cooler.
You guys are weird about the blue cooler. My red one has a nifty measuring thingy on the lid to tell me how big my mash is. 8)
+1 to not using JB Weld… New cooler is the way to go. No telling what influence that stuff will have on your beer, if any. But you’ll want to replace the thing anyway.
Buy a yellow one, just to upset all of the “blue cooler crowd”
A blue cooler guarantees that you get 137% efficiency.
You made the right choice. Sometimes it’s just not worth the effort to try and repair some things.
Wow
Yeah…it’s so efficient that you actually gain grain! After 10 brews, a 55 lb. sack magically appears in your garage!
Rats! I wish I knew this. I just retired my blue rectangular cooler for a keggle mash tun. 
It a god damn miracle!
Here’s the update. I got a 50 qt blue rubbermaid cooler, which will allow me to do bigger beers without doing a parti gyle. A copper pipe fits through a sloping drain port without leaking. So far so good. Next I have to build a manifold for fly sparging.
The 10-gal orange cylindrical cooler will be retired. It may see use for insulating fermentation. The existing manifold will be used in an old 5-gal cylindrical cooler which will be used for the weakest beers, basically just Berliner Weiss.