Hello Rolf my approach is to use epoxy resin at the first pore layer
first coat epoxy resin UV traitment (surf clear or other without pigment
second coat after one hours same resin with pigment (pigment for resin only)
in 3 another layer after 1h without pigment and not a thin coat because you ll sand
this 3 layer of resin give you a really nice deap effect and then after sanding with 320 paper and 380 you can apply the PU varnish also for an UV barrier