Washing a hot panel
Heat makes soap and water evaporate on contact, leaving mineral deposits behind. Let the vehicle cool in the shade before the first rinse.
Using household detergents
Dish soap and general household cleaners are not formulated for automotive finishes and can strip the protection you have already applied. Use a car wash soap designed for the job.
One bucket, one dirty mitt
Dragging a mitt loaded with grit across paint is the single biggest cause of fine scratches. Rinse the mitt often, swap to a clean one when it looks dirty, and save the wheels and lower rockers for last with separate tools.
Air drying
Letting a vehicle drip dry leaves behind everything that was dissolved in the water. Dry with clean, soft microfiber towels, working in straight lines rather than circles.
Letting contaminants sit
Bird droppings, bug splatter, and tree sap are acidic and can etch clearcoat over time. Address them as soon as you notice them instead of waiting for the next full wash.