How to weld polyethylene plastic on my kayak?

Question

do you need a special welder or is a propane torch ok

Answer 1

I assume this is a polyethylene (PE) boat.

Propane torches are too hot.

You can weld PE on PE in several ways.  The nicest way is with a hot-air plastic welding rig.  It blows an adjustable temperature of hot air to pre-heat the boat plastic and feeds plastic rod to fill in the weld.

Ski bases are PE also, so ski shops sometimes have little rigs like that.  But lots of shops just use the trick below:

The other, very cheap and simple way to do it is to pre-warm the boat with a hair dryer on "high" (so there is less temperature difference between the boat and PE filler materials.  For the filler material, you can buy PE rod, or just cut strips from a yogurt container or milk carton (check the recycling symbol on the bottom that it is PE, LDPE or HDPE).  Cut a strip about 1/2" wide and light one end with a match or lighter.  Molten plastic will drip off the end (like a candle held sideways) and you drip it into the cut or hole or whatever you are fixing.

For a biggish hole, put layers of masking tape on the other side (to stop and pool the molten plastic).  For any weld, orient the boat so the work area is level.

If you want to get fancy, you can find food containers or PE toys that are a similar color so the patch will look better.  Or just use clear containers.  There will be some soot in the patched area.  But hey!  What do you want for a repair that cost you a book of matches.

Wear leather gloves.  Molten plastic doesn't stay hot long, but it sure can burn skin.  Practice on thicker food containers or toys from Goodwill.  Try pre-warming and not and then stress your practice piece till it breaks.  See what works best for you.

Answer 2

It depends on what type of kayak/plastic you got and what you're trying to weld onto it.  If your going closed cell or rubber foam onto a modern plastic boat you use a contact type cement.  If you have a glass or wood boat you need a good 2 part epoxy resin.  If your going hard plastic onto hard plastic "gorilla glue" works O.K. but it is not a perfect bond.  Heat works well for some shaping and mending, but I have never seen a modern plastic kayak successfully welded with a torch, but I have seen it tried.  Contact the boat's manufacturer for their recommendations, modern boats have some really high tech plastics that may require a specific adhesive for what you're doing.  What are you doing?