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***DISCLAIMER***
I think it'd be pretty easy to destroy a boat trying to weld it. I probably wouldn't have started doing this except I had first watched someone do a weld and then experimented on a boat that I had paid $100 for and already gotten my money's worth out of. Even then, my first couple welds only held for a little while and had to be redone.
First, trim some plastic from an area that won't be affected by the loss of some plastic (I attacked some surplus plastic where the thigh braces attach). I figure if you use plastic from the same boat it will bond much better than any other sort of plastic. I tried P-tex, but it didn't work for me. I've heard of people using milk jugs, but I've never tried it and would guess that it wouldn't bond with a boat, but who knows. I use a utility/roofing knife trim thin stripes of plastic and make a little pile.
Then drill holes at the ends of the cracks (I let the boat's owner do that) to stop the crack from spreading. I make the holes at least twice as wide as the end of the crack.
One side note. The first boat I worked on was so badly oil-canned that one side of the crack in the hull was about a quarter inch lower than the other side of the crack. On my initial weld I tried to push them back together. I'm pretty sure this was a bad idea as the boat had already deformed and wanted to be deformed and thus pulled apart again in spite of the weld. Now I just weld the crack however it sits at the time.
I like to work on the inside of the boat first if that's possible. I carve the inside of the crack so that it makes a 90 degree V shape if you look at it in profile. This allows the weld to sit down in the crack.
Right before I start welding I clean the area with an alcohol swab. Then I heat the area around the crack enough that it's shiny while doing the same with a scrap of plastic that I trimmed. I use a heat gun and hold the strip of plastic with needle nosed pliers. I've also tried using a soldering torch, but find that it's harder to heat the plastic evenly and can also burn the outside of the plastic scrap, which will contaminate your weld.
It's really, really important not to overheat the plastic, especially the part that's still part of the hull. I try to get the outside shiny and flexible (plastic; pliable), but you definitely don't won't to make it liquid.
Next mush the scrap of plastic into the area you want to fill. I find that it works best to heat the scrap of plastic quickly and cool it as slowly as possible. I usually squish it into the holes first, then fill the crack. I use my finger to quickly squish it in. You want to do it quickly to minimize heat transfer because a. you don't want to burn yourself and b. you don't want to cool the plastic quickly.
If the scrap of plastic doesn't want to melt or feels brittle throw it away and use a different one. I think this is probably a result of either heating the scrap too slowly (or maybe it was overheated in the mold?).
Once I finish with the inside of the boat I fill in the crack from the outside. Then I take my roofing knife and very carefully trim and edges that look like they might catch on anything or any bits of plastic that look like they haven't melted thoroughly. Then all you have to do is let it cool slowly.
With this boat I found a piece of aluminum flashing (really thin roofing material that you can get from Lowe's/ Home Depot for approx. 40 cents) and gorilla glued it over the weld on the inside of the boat. My hope is that this will minimize the flex around the affected area. I'll probably cover that with a bit of duct tape just for good measure. Never used the flashing before, but I guess we'll find out if it works.
Good luck if you try this,
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