A sky burns in beautifully but the finished print shows a hard line along the horizon where the burning stopped. How is that avoided?

A. The card is kept in constant small movement so the transition is spread over enough of the print to read as a gradient

By keeping the card moving. A card held still casts a sharp boundary that prints as a line, and the line is far more conspicuous than the pale sky it was meant to cure. Constant small movement spreads the transition across a band wide enough for the eye to read it as tone rather than as an edge.