Bum pain occurs when the muscles of your buttocks are deprived of blood after being in a sitting position for a long time. It can be quite debilitating.
You need to find a system of prevention that suits you, so do plenty of long paddles in your training and work out a system that suits you.
Many of the experienced paddlers have found Ridge Rest and a piece of sheepskin to work well. I find it to be fantastic. Ridge Rest is a foam sleeping mat available from outdoor shops. An easy way to buy is online. It costs about $70 but could be shared by two or three paddlers. You can either stick it to the seat with contact cement or have it sitting on the seat–if you do this have a piece of cord attached to it in case of a capsize
The sheepskin will soak up sweat and is still comfortable to sit on when wet
I personally consider that Ridge Rest is an essential item for long distance comfort and have it in all of my kayaks, either expedition or marathon. I also use it to pad my rudder pedals and for my heels as I paddle in bare feet when possible. the foot comfort is fantastic when you do this.
[An alternative to Ridge Rest is a gel foam seat such as the Mirage 15mm Gel Foam seat - it is an adhesive seat pad that will stick to a wide range of kayak seats - ECK]