That's not entirely true... I tried to get prestine a while ago. You must not absorb at any time during the race, that's the important part. Anyway, race started, I did not absorb any weapons, just kept the one I got first which was leach beam. On the final lap I did get a shield reduction, I think I touch the side in a corner but since I was at the back of the pack I just fired leach beam on the dude in front of me, absorbed up to 100%, finished the race and got pristine. Most easy way to get it is to enter a newb race on Venom and just cruise at the back of the pack.