The final part of making a steel back nymph is to apply a coat of epoxy. This will protect the colour that you have applied to the steelback and most importantly adds strength to the stainless steel wires which hold the steel back in place. It is also possible to colour your epoxy which will speed the operation up. If you do this, be careful to add no more than 25% colouring or your epoxy may not set correctly. The proportions should be 50/50.
It serves no purpose to add additional hardener. In fact you may end up with epoxy which refuses to dry! I strongly recommend SLOW drying epoxy. You have more time to apply it and do any touching up before it sets. Slow drying epoxy is ultimately stronger than the fast drying product.
As for the instructions about using nail varnish, the paper makes the application of the epoxy much easier. Any excess epoxy should be removed while tacky. The very last operation is to remove the baking paper. This must be done BEFORE the epoxy starts to set. Use fine nose pliers to remove one side of the paper. The second side will pull away easily once the first side has been removed. Be careful not to catch any rubber legs in the process or you may break them off!