Let me start this by saying, I am far from an expert at repairing thermoformed kayaks. But I think my skills are now serviceable, in that I could do a repair in the field with a reasonably high level of success.
The boat I worked on was a Delta 15.5 expedition. It had severe cracks on both sides of the deck. As the boat had sat for 2 years, I started with a thorough cleaning. But it turns out I didn't do a thorough enough job. The edges of the cracks all had dirt in them that I couldn't get out. In the future I would dremel these cracks bigger for the sole reason of getting the dirt out. After cleaning them as best I could, I aligned the plastic. Most of this was done by hand, but some spots I used a rubber mallet to align the edges.
Once the edges were aligned I cleaned the entire area again, and then cleaned them a third time with Alcohol. I didn't think of this on the first section of repairs, but later on I started taping off the areas to be 'glued'. This made clean up a lot easier, and I was able to tape deck bungies out of the way. After it was taped it was time to proceed with the actual repair.
For this, you want Devcon Plastic Welder. I had a hard time finding it, and tried a couple of other items, but you really want Devcon Plastic Welder. The epoxies and glues just dry too brittle, and in the end we want a little flex. Mix your "glue" (I am calling it glue because that is what it feels like, but I am putting it in quotes because it isn't really glue) and use a disposable paint scraper to apply it to the crack. You will have about 3 minutes of working time. The amazing thing is that it will actually get hot to the touch - I was mixing it in a plastic paint cup, and could feel the heat through the cup. Make sure to fill the crack as that is what is really holding it together. Don't worry too much about what it looks like, you will be sanding it later.
After it has cured, cooled, and hardened it is time to move inside. This is where I had to improvise. I found a great resource on eddylines website, and they show using fiberglass tape and saturating it with plastic welder. Then applying it to inside of the boat, covering the crack. I couldn't find fiberglass tape, so I used the plastic mesh tape that is used to cover joints in drywall. It seems to have worked beautifully. You want to do several overlapping pieces, each covered with Devcon Plastic weld. When it is cooled, and hardened you are done with repairs.
Time to sand. I still have sanding to do, but it is a pretty simple, if time consuming matter. I started with an course sand paper and moved to a more fine one, but the trick is you want to wet sand this. Have a spray bottle handy and keep everything wet. It is easy, but as I said time consuming.
The only thing illustrated on the eddyline website that didn't work for me, was the dremel part. First, these boats don't crack in a straight line, all my cracks are pretty jagged, second, when I dremeled out the cracks, it really just splintered the plastic more. I may have had the wrong bit.
Another tip I would pass along, I did most of this wearing nitrile exam gloves, and it worked great until I touched the Plastic welder. A few cracks I used my fingers to work the weld in. But when you do this, you really have to remove the gloves right away, as it will melt the gloves off your fingers pretty quickly.
Another thing to keep in mind, is the sanding process takes the gloss coat off the boat. So after you are done the area around the repair wont have that beautiful glossy look. I haven't figured out how to return that to the finish.
If you know how, or have any other tips to make this process easier, please let me know. I am sure there are more skilled people out there, so I look forward to your tips.