Is there a smoother type of concrete for fingerboarding curbs or tips for making your concrete curb super smooth ! ?

Lacquer it. It makes the curb super smooth and you wont need to wax it either

If you want to make your own super smooth, then you need a real plastic mold. I made on and it's actually shining.

you can lick it smooth.

It's very very smooth when you mix water with cement and fill it in plastic mold then.

imo it helps if the edges aren't rough so you should get a plastic round mold

Masonry cement is your best but for a super smooth cement. I've made ledges and my cement bowl out of masonry cement.

2 parts plaster 1 parts Resin (Pre mixed) ... add food colouring for desired colour once set as hard as rock.. but smooth like glass... (when done right)... hardest part was bubbles... i used this concoction in my last year at collage (way way back in the day) for making sculptures. cant see why it couldn't be used for Fb use

On my Bonsei Barrier, I just use my wax.