The minimum thickness of a steel plate, which is directly exposed to weather and is not accessible for cleaning and repainting, should be 8 mm. This is because steel plates that are exposed to the elements will corrode over time, and the thicker the plate, the more resistant it will be to corrosion. Additionally, steel plates that are not accessible for cleaning and repainting will accumulate dirt and debris, which can also accelerate corrosion. Therefore, it is important to use a thick enough steel plate to ensure that it will last for many years without needing to be replaced.
Option A, 4.5 mm, is not thick enough to provide adequate corrosion resistance. Option B, 6 mm, is a bit thicker than Option A, but it is still not thick enough to provide adequate corrosion resistance for a steel plate that is directly exposed to the weather and is not accessible for cleaning and repainting. Option C, 8 mm, is the minimum thickness that is recommended for a steel plate that is in these conditions. Option D, 10 mm, is thicker than Option C, but it is not necessary for a steel plate that is in these conditions.
In conclusion, the minimum thickness of a steel plate, which is directly exposed to weather and is not accessible for cleaning and repainting, should be 8 mm.