Mounting panels on galvanized steel structure
I would suggest using an off the shelf clamp rather than a washer. These clamps have been tested to withstand the wind loads a solar array will see. A washer hasn''t. For grounding, make sure you use a tin plated copper ground lug such as a WEEB-LUG or CL501TN. These are designed to be used on either galvanized rails or aluminum module frames.