Thank you to my wonderful curriculum director, Kim Lysne, for this tip! She hadn’t tried it, just heard about it. I told her NO WAY would this work. But it did! It’s a good day.
- Write over the sharpie with an Expo maker.
- Erase.
MAGIC! Yes, it’s that easy! I wouldn’t have believed it if I didn’t see it for myself so I just had to share!
I Speak Math 
I learned this the hard way after I purposely drew a grid on a whiteboard with a sharpie so I would have a “graph paper” whiteboard for demonstrations. The first time I used it (with Expo markers), the grid started disappearing.
I had a student grab a sharpie one time (mistakenly) and wrote on my board. Luckily, we figured out the dry erase marker tip. Phew.
Another fun trick is that WD-40 actually works to revive whiteboards. My 10+ year old board is now working like new–easy to erase and no smearing.