Here's a video of a kayak that had no obvious signs of damage however it took on water every trip especially when it was choppy or rough.
Turn your sound up and have a look.
Cheers from Yakdoc.

This kayak had some issues and just as in life, things didn't go to plan, this is part one of the video and part two will be added in the near future.  Turn your speakers up a little and see what's happened so far.

Cheers from Yakdoc.

Kayak looking a little scruffy and scratched? its normal if you fish from it.
you can make it look a lot better with "careful" use of a hot air paint stripper gun, warning!! warning!! warning!! do not let the hot nozzle of the gun touch... your kayak!! it will melt it instantly.
Watch the video  of me cleaning up my kayak, the scratches have a small fluffy ridge of plastic either side and the hot air gun fuses these ridges back onto the hull, be warned it takes a long time to do the whole hull and it must be scrubbed clean before heating.

Don't stay in one spot for long, do one part then move to the other end on the other side, once again, clean it first!! keep the gun moving!! keep the gun off the hull.

All of the damage you see above is from scupper carts, the type that go through your scuppers,  and all the weight of the kayak and gear sits on those two small surfaces.
The cart twists inside the scuppers when you go over uneven ground or thick sand placing a lot of strain on what is in my opinion, the weakest point of most kayaks I have seen so far.

I am dead set against scupper carts and wont use or recommend them, the scupper tube wall thickness leaves a lot to be desired in the kayaks I have repaired to date, some of them are less than 1mm!!.

The type of cart I recommend and use myself supports the kayak over a much greater area and offers far more support, sure you can't just "bung it on" and get going however it doesn't take long to get used to strapping a cart on.

This is probably the best deal I have seen so far on a good, lightweight, sturdy kayak cart that will give much better support to your kayak, its available from JAG Trading Online on E-bay.