Looking closely at the photo you sent I believe you have leaf miners not sunburn. A small black fly lays eggs on the leaf and the larvae burrow in making blisters from the tunnels. Remove every leaf that is infected to keep the larvae from spreading. The only real practical cure is to cover the crop with fine screen or Remay agricultural fleece to keep the flies from landing on the plants and laying their eggs. Place the fleece over seedlings as soon as the seeds for the next crop sprouts. Good luck and Keep growing, Marianne Binetti