Warhammer 40,000: Kill Team is the best place to start for the 40K
How to Prime Miniatures with a Brush
WIP it good: first time using brush-on primer – Yore
Build & Paint
The Army Painter Warpaints Air Primer Matt Grey 18ml
Warpaints Air: Desert Yellow
Vallejo Surface Primer for Miniatures (Review and Tips) - Tangible Day
WIP it good: first time using brush-on primer – Yore
Air brush primer is easily scratched of model with fingernail
6 Ways to Use Primer When Painting Gunpla - Gunpla 101