2 of 3.5 oz Detailing Clay.
Even after you washed your car thoroughly, you still feel scrachy bumps in your palm. They are micro size metal fragments that embedded to the paint layer. So many metal fragments are flying around streets. Using a clay bar will remove embedded surface contamination.
There are chemical solution to eliminate micro metal from paint surface, but, thoes are heavy chemicals and Clay is safe to use and eco-friendly.