On their site, they say the regular stuff is resistant up to 500 degrees F constant (or up to 600 for a few minutes, like 10 mins). The "high heat version" is resistant up to 450 constant. (I've never seen a temporary heat rating for the high heat version.)
So ... I called them up and got clarification about the apparent (to me, at least) discrepancy. Here it is, from J-B Weld Customer Service:
The High Heat version is the highest-heat stuff available from them as a PUTTY, whereas the regular J-B Weld "we all know" is functionally just as good with regard to heat resistance if you want instead something easier to spread.
... And there are differences in cure times; the high heat putty cures much faster, but for the sake of comparisons here let's assume you have plenty of time to wait for curing so long as the fix works as you intend.
... And there are differences in PSI strength, although the company doesn't say what happens to either product as it approaches the heat limit--unfortunately they don't publish the curves. We do know they'll both turn into goo when too hot, or even smoke/burn up, but they don't say what happens to the PSI strength when hot-but-not-hot-enough-to-melt.
For example, the high heat PSI rating is ~800 and regular J-B Weld is more like ~3900, but at what temperatures? 400 degrees F inside a kettle is easy to achieve, below the melt point of either, but what's the strength there? That could come into play for hot leg sockets.
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Applications:
One of my 26 flat tops, ('90s, not "vintage") has both of its metal bowl handles a bit more loose than they should be. Not about to fall off, but can be "bent" or moved a little by hand, you know? There's no damage to them; the welds are just kinda shitty probably. And rusty now. I want to sandpaper and then slather the connection points with regular J-B Weld. I wish the metal inside the bowl at these areas was thicker.
I have an 18 with slightly loose leg sockets and a very small hole (1/8" or smaller, almost a pinhole) in the bottom of the bowl. Based on what I was told today, I'm not sure the high heat version of J-B has advantages here either.
Your thoughts?