I throw them away. Life is too short and fishing time too precious to be welding plastics so they'll stay on a hook.

Im not sure but I think the strong burnt plastic odor repaired baits have is offensive to fish. I didnt seem to get bit as well when I did it.

I bought a tool once that was made for remelting soft plastics. I has a point that heats up and you remelt the baits back together. I have reused baits like lizards, crawtubes, flukes, worms and other baits as many as 3 to 5 times each. I used to melt a lot of baits back together and donate them to kids to use. I think I got the tool from Carl down at Amistad a few years back, but you can also just use a lighter or small soldering iron.