Glossy finish varnish

In the house that we live in, the former owner has used some kind of heavy duty glossy varnish and it is beautiful to look at and feel.

Over the years the bits of the inside window sill that has been exposed to the sun have developed cracks in the varnish surface.

I asked in the local paint shop and was recommended yacht varnish.

Sanded the surface, wiped it clean with some wet wipes, allowed to dry for a day and then applied the yacht varnish. It has not come out like the original surface. The new surface is only marginally glossy in comparison and more importantly I can feel a granular surface like someone has spread sand after applying varnish.

What could ua w gone wrong ? Could it be the varnish or does the varnish need an additional treatment to make it hard and super glossy. Thank you.