Performed a heating pm on an oil boiler. Brushed and vacuumed. Replaced the oil filter, nozzle and strainer. Started and checked the safeties. Performed a combustion test. 4.9% o2, 29 ppm, 0 smoke System is operating properly. While performing the maintenance we found the corrosion from the hot water coil has spread farther onto the boiler. The corrosion is now starting to form on one of the boiler clean-out covers. Could not remove 1 out of 3 clean-out covers. This section of the boiler will begin to get plugged with soot because of this and will eventually lead to performance issues. Showed the customer the corrosion on the boiler. Recommend replacing the boiler soon to avoid any issues with the corrosion.