I did not sand mine. I did wipe the inside down with a wet rag first to get any dust or any dirt, then went back and wiped again using alcohol to get any greases that I couldn't see. Then I used 3 cans of flat black paint and then went over that with 3 cans of dupicolor. The dupicolor was more expensive than the paint and once you have the black base coat, you don't need to completely coat everything 1/8 thick with bedliner.
You could probably do the whole thing with 4 cans of spray paint meant for plastic if you wanted. It wouldn't be as rough to the touch.