Had this incident while paddling one fine morning where the plastic edge of a wooden Dragon Boat Paddle was damaged where the whole corner chipped and broke off.
After some thought, decided to try restoring it both functionally and aesthetically, with rather pleasing results!
Both visually and by touch, the paddle after repair was indistinguishable from the other undamaged ones. Also water tested it over 5 sessions and the repair showed no signs of degradation or any visible problems.
Step 1: Prepare the Damaged Area
Use a knife to scrape or smoothen out the remaining plastic of any sharp corners or crack initation points. File down any sharp cornners where stress could concentrate.
Make a 45 degree taper downwards, so that our restoring filler material can adhere to the original plastic better.
Step 2: Find Suitable Reinforcement Material
For this case, you could use an old piece of nylon netting, gauze, or even fibreglass or carbon fibre cloth if you can get your hands on some.
This would add reinforcement to our filler material and reduce likelihood of it cracking in the future.
In this case, I cut out the nylon netting from the side of a cheap zipper pouch. You may also use the carbon fibre cloth as in image 2, but i found it easier to work with the nylon net.
Step 3: Mix Some Epoxy Putty
This is easily available at various hardware stores in various grades. Mix in equal parts of the A and B putty well.
As the color may be different, mix in some pigment ink or paint to match the original colour of the remaining plastic. (black in this case). Knead and mix it in well till the colour matches.
Step 4: Patch the Paddle With Reinforcement Material Embedded
Use the putty to fully fill in the gaps and form thin layer on the damaged area of the paddle. Make sure it fills in any gaps in the previous broken shard of plastic as much as possible.
Lay over the reinforment material on top of this initial layer of putty.
Step 5: Patch Up the Rest of the Damage
Patch up the rest of the volue of the damaged area with the rest of the kneaded coloured epoxy putty.
Do not worry about exactly matching the original shape yet. Slightly larger volume is desired so that it can be filed and sanded to shape later.
Step 6: Wait for 24hours or More to Cure.
Refer to the instructions on the back of your epoxy putty.
Step 7: File and Sand the Repaired Area to Shape to Match the Remaining Plastic
Use a fine file, or better a sanding block.
Gradually sand with 150 to 400 grit sandpaper in increments to match the original profile and flush with the remaining original plastic.
Step 8: Optional: Lacquer to Seal / Finish
Spray a few coats of clear lacquer over the entire surface including the original plastic to give an even finish.
Paddle edge looks as good as new and the broken part is indistinguishable!