The article presents imperfections tested using penetrant methods. The research described in the article was  concerned with imperfections regarded as the most hazardous, i.e. cracks. The tests involved the formation of natural cracks in structural steel S355J2C+N. The cracks were measured in terms of their width and the roughness profile of their surface. The research involved a number of penetrant tests aimed to determine how a given factor (crack width or crack surface roughness) affected the time of development in penetrant tests of joints made in structural steel.