What would make it harder to remove penetrant from a crack when using a spray water wash?

In the context of liquid penetrant testing, high viscosity of the penetrant can significantly impede the removal process from a crack. Viscosity refers to the thickness or fluidity of a substance. A penetrant with high viscosity is more resistant to flow, meaning that it adheres more tenaciously to surfaces within the crack. This tenacity makes it difficult for the spray water wash to efficiently dislodge and wash away the penetrant, as the high viscosity prevents it from easily being lifted by the force of the water.

When a liquid penetrant has a low viscosity, it flows more readily, allowing the spray of water to better displace and remove the penetrant from the cracks during the wash process. Therefore, high viscosity is a critical factor that determines how effectively the penetrant can be removed, particularly from tight or intricate areas like cracks.

Thus, the high viscosity of the penetrant directly contributes to the challenge of achieving a complete removal with a spray water wash, making it the correct answer to the question.