This page outlines how to fix a crack or deep scratch that penetrates the hull of a plastic kayak. Other types of plastic kayak damage usually don't need repairing.

If you have a damaged kayak, before you start repairing you should ensure you know exactly what material it is made from (click here for details) and you should make sure you have assessed the damage to decide which type of repair to do. (click here for details).
 
Materials:
Drill (either electric or hand-powered)
Rough sandpaper
Flathead screwdriver
Heat Gun
HDPE (High Density Polyethylene) Plastic cut into fairly thin, narrow, long strips. It doesn't matter if you use uncolored or colored HDPE. The only reason you would use colored is if you want to match the color of your boat.
An Old Spoon
(Optional) HDPE cut into small, ultra thin squares.
(Optional) Surform shaver

Step 1: Stop the crack from spreading:
a) Drill each edge of the crack so that at each end of the crack there is a small (and stronger) circular hole. See image below.



Step 2: Prepare the crack for welding
a) Pry open the crack enough to slide sandpaper inside. Be careful not to cause the crack to grow.
b) Sand inside the break, roughening the surfaces. This helps the weld stick later.

Step 3: Open the crack ready for heating:
a) Using the screwdriver, pry the crack open. Once again be careful not to cause the crack to grow.
b) Leave the crack slightly pried open ready for heating with the help of the screwdriver.

Step 4: Using the heat gun, heat the cracked area:
a) Heat the inside of the crack to the point that the stressed, white plastic starts to turn back to the color of the boat.
b) Heat the general area of the crack so it is just warm enough to touch with the inside of your wrist. See image below.



Step 5: Preheat the HDPE:
a) Hold one end of one of the strips of HDPE and heat the other end with heat gun until it is very flexible and almost runny.
b) If you are using uncolored HDPE it will be ready when it starts to go transparent.
c) If you are using colored HDPE it will be ready when it starts to become glossy.

Step 6: Start the plastic welding:
a) Further heat the HDPE and start to gradually work it into the crack. See image below.
b) Blend the two plastics using the spoon as you go.
    There are a few things you need to be careful of in this step:
    1. Keep the boat hot enough so that the it will accept the repair and meld with           the HDPE.
    2. Don't let the boat get so hot that it starts to melt out of shape.
    3. Don't heat the boat too fast so that it starts burning (going brown).


   
Step 7: Weld the entire crack, using as many pieces of HDPE as necessary.
a) Be careful about boat temperature as you go. Make sure the boat stays hot enough, but not too hot.

You can finish your repair here, once the crack is welded. The structural repair will be completed. If you choose to do this, let the boat cool for at least an hour before trying to do anything with it.

However if you want to reinforce the repair continue to step 8, below:

Step 8: Reinforce the weld:
a) Using a similar technique as in step 5 and 6, melt the ultra-thin pieces of HDPE over and around the repaired crack. This provides reinforcement. See image below.



Step 9: Allow the boat to cool for at least an hour.

Step 10: Smooth the repair (if you want to):
a) Warm the repair area.
b) Grate off any raised or sharp areas using the surform shaver. Don't try to do too much. Your repair will never look perfect.


Pictures and Information from totalwhitewaterfun.blogspot.com
For specific link click here