Just make sure you buy Chrome Oxide formulated or tested for razor use, as pure as you can get.

Woodworking or hardware store Chrome Oxide rarely contains pure Chrome Oxide and is often sold as Green Compound because it has so little or no Chrome Oxide, which accounts for many complaints of harsh edge. It’s ok for tools but not razors.

SRD Chrome Ox is good paste, inexpensive and a lifetime supply.

When applied, you do not need much, apply sparingly for best results.

You should not need to flatten your balsa, do not use sandpaper, you will just introduce sand paper grit into your strop. Chrome Ox is 30k grit, your sand paper is not even close.