For the TL;DNR crowd here are the pictures.
https://imgur.com/a/U3F1gKd The first is the repair - reinforced with stainless steel mesh. The second shows the final step - closing up the 5/16 drain hole I drilled in the top of the tank. In the later you can see where some of the black substrate has been stirred in while forming the blob.
if you are looking at opening-up the crack to heat the edges, wouldn't it be better, and easier to heat the edges by grooving into a 'V' the crack? Seems like you would be able to heat more of the plastic needing repair, providing a better bond between the filler rod and the tank.
The thing you add plastic to is the substrate. From my piddling practice tests on old and new
HDPE (Hi-Density-Poly-Ethylene) plastic buckets, I came to the conclusion that NOT overheating the substrate was a problem with the buckets and a probable problem with tank. The material you're adding is essentially the same as the material you're adding to. When HDPE plastic "melts" it slowly. First it gets more flexible - then mushes very slowly - still pretty slick but becoming self tacky - then eventually a sticky pile of goo — and ... it's warm, runny syrup.
OK, I watched the video, it doesn't look like he spends much-time heating into a plastic (soft) state the barrel material. He directs lots of heat to the rod he's using, but I think his long-term success depends upon more-heating of the barrel material before introducing the patch rod. Sure, it looks like the patch rod is adhering now, but it's probably not a good bond between the rod and the barrel.
He puts enough heat to it to melt the surface of the substrate. Then he scrapes away any debris caught in the outer layers exposing plastic less subjected to oxidation and out gassing.
When he begins forming a molten blob of plastic, most of the gun heat is directed at the strips, but the already warm substrate is still absorbing overflow heat from both the gun and the blob of hotter plastic on it's surface.
When he has decided that the blob of plastic is big enough, he does two things:
- He mashes the molten plastic that is beginning to become self sticky but is still slick to the metal surface of his (homemade?) pressing tool.
- Some or all of the gun heat is now being directed in the direction of the blob, the substrate it is on, and the mashing/pressing tool. All three get hotter.
As the blob gets mashed thinner more heat is transferred from heat gun to blob and substrate; and from the blob to the substrate at the same time. If the blob is hot enough, a bond is form all along the interface between the blob and the substrate it's pressed into. Base on having tried to peal the new stuff off the old stuff with a dull wood chisel, a moderately sharp pocket knife and a single edge razor scraper, I think this method forms a good bond and can be stronger than the original. I'm convinced this is a good structural repair. Time will tell, hopefully not a lesson in humility.
If you're planning to use this on a gas tank, putting more-heat into the gas tank material before introducing the rod I think would provide better bonding, and make it more-resistant to vibration failure down the road. I'd do it with a charged hose at-hand, outside, and a 5 or 10 lb ABC dry chemical extinguisher too.
The gas tank substrate is only about an 1/8 thick - maybe a tad less. In some ways this was like trying to braze with a cutting torch.
I played it safe - outside, extinguisher ...
