I use 4,5mm thick spruce strips, after sanding between 3,5-4mm. With double glass layers outside and a part double inside, I lay the inside with overlap on the keel area in cockpit, outside filled and only the area inside where you put in your feet filled. With this layup I can reach maybe 6mm if I´m very generous with epoxi. The ShootingStar I built for my daughter weights in at 17 kilo with this layup and you could drive a truck on it. I think that I could have got her well under 15 kilo if I had worked on it.
The shores in the lake where we paddle the most is covered with debris from old copper- and ironmines. It´s very sharp and hard, like obsidian-glass. You get some scratches but the laminate are very durable so you have to be very careless to get severe damage to the kajak.
I think that the ´footarea` in cockpit are important to have extra epoxi and glass on. If you have sand on your feet when entering you will soon grind a hole in the laminate.