Most of the new (easily obtainable) battery erasers use 5mm refills. Even the Helix machines has changed from 4 to 5 mm. The problem is you only get the soft in 5mm. Untill now. I found these Seed Sun Dolphin ones at Jet Pens. They are quite a bit pricier than the regular soft (graphite) ones. but you do get 60 in the bag and they are very hard wearing. The regular soft ones compare to the coarse ones. There is a bit of an issue though. They not not fit 100% in the gripper. A new fresh refill is not a problem bu if it gets shorter the gripper does not have enough bite. So you can make a small adjustment to the gripper. Just bend the edges a bit inwards. You really do not to do much bending at all. This is the modified one compared to the original one. Then I did a comparison to see how good the erasers are. I tried various paint on cheap blending card. Paints were straight from the bottle with thick 100% coverage at the bottom. I think the results were great.