I know how the plastic welding is done, but I’ve never attempted any patch, so I don’t know enough about the process to offer a fair assessment. However, several points highlighted in yellow, red and oranged raise a concern. I’m curious if anyone can offer suggestions about the repair.
the yellow area looks weld may have been too cold to properly bond.
The red highlighted area looks like a patch on the patch.
not sure what is going on with the orange area, but that is the least concern.
This makes me feel a lot better about my repairs!
The kind of dent the op’s seller meant was probably not a push-in dent like a fender-bender, but probably where something took out a divot. I put a “dent” in one of my kayaks when it slid out of position on a trailer and came to rest against a spinning tire, which quickly wore through the outer layer and most of the way through the core layer. I used a heat gun to meld in a piece of polyethylene and it floats great.
I’ve learned through another repair I once did that if a repair isn’t perfect, you get a slow leak, so you have a chance to discover it and to do it again instead of having a sinking disaster.
