Last step in making non-slip area's.
After the varnish has cured you can start brushing loose sand and smaller stones off . Do not use a very soft brush, but also avoid to damage the fixed layer of sand.
Use a vacuum cleaner to remove things carefully.
The last step is a second layer of varnish on the sand to fix it.
This sand will soak up quite a lot of varnish, I have used some extra thinner to be able to spread it better. I also varnished the masked area's to have a smooth layer on both bottum panels.
It can take a few days before this last layer of varnished has fully cured.
This leaves a very functional semi-transparant non-slip area with great durability. If it appears to be too sharp, you can use a second layer of varnish.