Sandpaper grit progression is a process that is used in the auto body trade to prepare a metal body part for painting. Sandpaper grit progression require that the body worker use sandpaper of decreasing grit from coarse to fine sandpaper. By using sandpaper of each grit level in sequence, the body worker can remove the old paint, rust, and create a smooth surface for painting.
By skipping sandpaper grit level, the body worker will leave deep scratches in the metal that will be visible when the part is painted with new paint. The first stage of sandpaper grit progression is to use coarse sandpaper. Coarse sandpaper remove the old paint and rust layers from the metal part.
How to Sand a Car Part for Painting
An random orbital sander can assist in removing this old paint and rust. Coarse sandpaper will create deep scratches in the metal that the body worker will fix later in the sandpaper grit progression process. After using coarse sandpaper, the body worker must perform a process known as featheredging.
Featheredging use a sanding block to create a flat metal part. If the edge is not feathered proper, the old paint will be visible at the edges of the metal part after the parts are painted. Sandpaper grit progression also includes a shift from dry sanding to wet sanding.
Dry sanding is used early in the sandpaper grit progression. Dry sanding allow the body worker to see the high and low spots on the metal part. Wet sanding use water as a lubricant.
Water will help to remove metal debris from the sandpaper, and will also cool the metal part being sanded. Wet sanding produce a fine sandpaper scratch pattern on the metal part. Wet sanding will also prevent the sandpaper from getting clogged.
To properly sand the metal part with sandpaper, a specific pattern must be used. The metal part should be sanded in cross-hatch strokes. Cross-hatch sanding involve sanding in a diagonal direction on the metal part and then sanded in the perpendicular direction.
Cross-hatch sanding will help to erase the scratch pattern from the previous grit sandpaper. Additionally, a guide coat must be used between sandpaper grit level. A guide coat is a dark powder that is sprayed onto the metal part.
The guide coat will settle into the low spot on the part so that the body worker can see them. Not all tool are created equally during the sandpaper grit progression process. Longboards are used to sand large flat metal area, like a hood.
Longboards will help the body worker to find the waves in the metal part. Additionally, foam interface are used to sand the metal part in curved areas. Foam interfaces will flex to accommodate the metal part curves.
Finally, sandpaper disc must be changed often during the sandpaper grit progression process because sandpaper discs become loaded with metal debris. If there is defects in the metal part being prepared, the body worker will use the correct grit of sandpaper to fix those defects. For instance, orange peel defect can be fixed using wet sanding and fine sandpaper.
If there are runs in the paint, the area must first be filed, then blended using the sandpaper grit progression process. If there are edge breakthroughs, the part must be taken back to the feathering stage to fix the defect. Each defect will require the body worker to use the correct grit sandpaper to fix the error.
Sandpaper grit progression is a process that take the body worker from coarse sandpaper to fine sandpaper to prepare an auto part for paint. Coarse sandpaper is used to remove the old paint and rust from the metal part. Featheredging ensure that the edges of the metal part are flat and that old paint will not be visible on the part after it is painted.
Wet sanding ensure that the part is finished to a fine sandpaper grit that is ready to accept the primer and paint. If the body worker follow the sandpaper grit progression process, the metal part will be prepared and ready for primer and paint.
