Purpose The first in first out (FIFO) rule is an indispensable rule in the automotive industry. However, during production, the order of coming to welding operation and exiting through the painting operation is broken. Consequently, the rule of FIFO also gets broken. The purpose of this paper is to eliminate the deviations of the automotive painting process in a just in time (JIT) environment. Design/methodology/approach The paper has been solved using observations and expert’s opinions and recorded data in the real problem area. Bypass technique and express line have been used as a real problem for the minimization of these deviations. The current process and the bypass process have been compared with simulation optimization. Findings This paper provides analytical insights about how to prevent delays during the JIT production painting process. Delays have been prevented by using the bypass technique and express line together. The number of delays reduced with an improvement of 129. The ratio of effect (for four months) has been founded to be 0.12. The output rate has been increased from 80% to 89%. Research limitations/implications This paper focused on the number of vehicles in the cooling area of the painting process in the study; there may occur bottleneck processes or products. It does not contain station-based bottleneck processes. Practical implications From the real-case problem, one will see that the bypass technique and the express line can be applied and generalized to many similar problems and help companies conduct their assembly lines more efficiently. Originality/value This paper is the only example that has been used to bypass technique and express line together on the paint/cooling process in a JIT production system.