I think your ok doing what you've already done(duck tape)ive used this on my rails and actually double layered it,this doesn't cause me any problem,my trouble area is the nose and tail when moving the board about on dry land,i'm thinking about cutting up a bike innertube and glueing it to the nose and tail to form a rubber end cap but i'm not sure wether it will look crap, may offer abit up before gluing.

Sweden

Mate I just started riding a ten foot C4 and after the first surf I noticed a few little chips on the rails,so I bought some clear waterproof tape and applied that to the rails after my next surf I noticed that even though it helped I still wasn't happy as there were still one or two small chips .So I went to "Super Cheap" Auto parts and accessories and found some thin clear stone chip tape used to prevent stone chips on various parts of your car, It comes in a 200mm x 1600mm roll for around fifty dollars . I used a straight edge to cut it into three strips and ran it down the rails ,its pretty easy to apply, just take your time and make sure you work from the bottom of the rail up and get all the air bubbles out you can prick any bubbles and then rub them out with a cloth Once its on its on!! so be careful. anyway yesterday I went for a paddle in crappy cross swell on shore conditions and it worked a treat " no more chips " I think it unavoidable that your paddle comes in contact with your rail at some time during a surf . I'm not sure weather you have "Super cheap" in Sweden but I'm sure you will find it somewhere.


Cheers mate hope this solves your problem

Hey Sweeden,
If you want to do top notch job on your gong, go find a auto parts supplier and buy some "stone guard tape". Its the stuff like companies like SAAB put on the cars behind the wheel guards to stop stone chips. Its a thick clear tape that can be bought in rolls and cut to the shape of you rail.

Its added to this board and as you can see, you cant see it at all.
Lasts for ages.
Regards,
Scott