How to get a thick layer of polyurethane on a table?

You have to keep applying the resin but be sure to lightly sand and clean well in between each coat.

I don't know if you can put poly over varnish, it is usually applied over the stain or paint. If you do use it, you need to use fairly thin coats, just do multiple coats, following the directions for time between coats. If you go too thick at once, you could have brush stokes, or uneven thickness. I have a table my father refinished and used poly on it and used three or four coats and it still looks like it was just refinished. He refinished the table in the seventies to use in the family room in their basement. I have used it now with my family for about seventeen years.

2 part epoxy will do the trick. Read the instructions