Hard wax seal experiences - What to pay attention to?

I have now ordered the Remmers HWS 112. The carpenter said he would have used it as well.

One more question:
Would you treat the washbasin board only on the top sides or also on the underside? Or is it advantageous if the wood can still "breathe" on one side?

Both sides. Nothing needs to breathe there.

I looked at the datasheet. Well-suited product. Everything done right.