And I'm younger by another 6 days

Since you have a multitool, use the coarsest attachment you have to get rid of the top coating. The wire brush fitting might do the trick.
Then use one of the sanding stone to finish off your shape, and get the last of the coating off.
When sanding you want to run the paper along the length of the pick. Get all the scratches going in the same direction. Try to avoid swirling motion if you can help it.
Start with the lowest number paper you can find. Sure it'll make some deep scratches, but your first step is finalising the shape of the pick, and getting it flat.
Then as you're sanding, watch the scratches, and look for the larger scratches disappearing.
Then step it up to a finer paper.
You want to work your way up to 1200 or higher paper to finish off the pick. Sure, you can jump straight to the 1200, but it'll take forever, and you'll go through heaps of paper, too.
Try 1200 while dry, then add water.
400 is just too coarse to get a nice finish.