Block sanding is a meticulous process used to create a perfectly flat surface on a car's body panels before painting. It involves using a sanding block to evenly remove a thin layer of primer or existing paint. The "block" is crucial because it prevents your fingers from creating uneven pressure points, which would result in waves and low spots. For a professional-quality finish, proper technique is essential.
Safety is your first priority. Wear a respirator mask, safety glasses, and nitrile gloves. Work in a well-ventilated, clean area to avoid dust contamination.
You'll need a few key tools:
- Sanding Block: A rigid, long-block sander (around 12-15 inches) is ideal for large, flat panels.
- Guide Coater: This is a spray-on powder that helps you visualize high and low spots. As you sand, it disappears from high areas but remains in low spots, guiding your work.
- Sandpaper: Use wet/dry sandpaper designed for automotive use. Starting with too coarse a grit is a common mistake.
The process begins after you've applied a coat of primer-surfacer. Spray on the guide coater. Then, using your sanding block, sand in straight, overlapping passes along the length of the panel. Keep the block flat and use a spray bottle to keep the surface lubricated with water; this is called "wet sanding" and it reduces clogging. The goal is to sand until the guide coater is uniformly removed, indicating a flat surface. You'll typically progress through grits, starting with 400 or 600 to remove imperfections and finishing with 800 or 1000 for a smooth base for paint.
| Sanding Grit | Primary Purpose | Recommended Use |
|---|
| 320 Grit | Aggressive material removal, shaping body filler | Initial heavy work, not for final primer sanding |
| 400 Grit | Removing minor scratches, leveling primer | Starting grit for block sanding on rougher surfaces |
| 600 Grit | Final leveling of primer, removing 400-grit scratches | Standard final grit for most base coats |
| 800 Grit | Creating a finer scratch pattern for paint | Used for high-solid primers or as a final step |
| 1000 Grit | Ultra-fine finish for sensitive paints | Often used for clear coat or single-stage paint |
| 1500 Grit | Final pre-paint polish preparation | Used before applying a polish for a show-quality finish |
