how to sand car for primer

[LaSabrina]

Sanding a car for primer is about creating a perfectly smooth, clean, and slightly textured surface called a mechanical bond for the primer to adhere to. The most critical step is using the correct grit sequence, starting coarse to remove old paint and imperfections, then moving to finer grits to eliminate the scratches left by the previous sandpaper. Proper technique, like using a sanding block to avoid uneven pressure and frequently cleaning the surface, is essential to prevent contaminants from getting trapped under the new paint.

The process begins with assessing the car's current condition. For bare metal, start with 80-120 grit to remove rust and shape any filler. For existing paint in good condition, 320-400 grit is sufficient to scuff the surface. Always feather sand the edges of any chips or old repairs to create a gradual transition. After the initial sanding, switch to a finer grit, like 400-600, to remove the scratches from the coarser paper. This feather sanding process ensures no visible lines under the new primer.

Using a dual-action (DA) sander makes the job faster and reduces the risk of creating swirl marks, but delicate areas and edges must be done by hand to avoid cutting through the paint. After sanding, the surface must be meticulously cleaned with a wax and grease remover and tack cloths to eliminate all dust and oils. Any residue left behind will cause the primer to fisheye or peel. The final surface should feel smooth to the touch and have a uniform, dull appearance with no shiny spots.

Sanding Stage	Grit Range	Primary Purpose	Tools & Notes
Heavy Material Removal	80 - 180	Strip old paint, shape body filler, remove rust.	Use with a DA sander; avoid on edges.
Feather Sanding Edges	220 - 320	Blend repair area edges smoothly into surrounding paint.	Done by hand with a sanding block.
Final Surface Prep	400 - 600	Create a uniform scratch pattern for primer adhesion.	Can use a DA sander; always finish by hand near edges.
Scuffing Clear Coat	600 - 800	For painting over existing clear coat (basecoat/clearcoat systems).	Light scuffing only; do not sand through.
Sanding Primer (Post-Application)	320 - 600	To level the primer coat before applying basecoat.	Wet sanding is often recommended for a smoother finish.

[SanJesse]

The key is the grit. You can't just grab any sandpaper. Start with something like 320 grit if the paint's decent, or 180 if it's rough, to get rid of the gloss and scratches. Then, you have to move up to 400 or 600 to make it smooth for the primer. Always use a block so you don't get finger grooves, and wipe it down with a pre-paint cleaner before you spray. Miss that step, and the primer might not stick right.