We consider rough products produced by a factory. Each product coming from the plant has m vital elements and some elements can be damaged. To obtain a perfect product (i.e. all the constitutive m elements are safe) all the damaged elements are repaired and a test phase follows. The result of this two-steps procedure is random. We suppose that the number Zk of non-damaged elements is a Markov chain valued in the set {0,1,...,m}, where k is the number of applied repairing-test phases. We have a qualitative result which says that if the repair phase is efficient then P(Zk=m) is close to 1. As for production of a large number n of products, the former result allows us to give conditions under which either the n elements or a fraction of these n elements are (is) safe after the application of k previous maintenance phases.