Proven Quality
400 vs 500 Grit for Primer: Read the Topcoat Specification
The choice between 400 and 500 grit on automotive primer belongs to the coating system. A change of number can alter the final scratch, but it cannot correct undercured primer, contamination, or a poorly leveled repair. Read the primer and topcoat instructions together before deciding.
Find the right row in the data sheet
A technical sheet may list preparation of old paint before primer, sanding of the primer itself, and preparation for different topcoats. Those are different operations. Copying a familiar grit from the wrong row can produce an inappropriate surface even when you follow that number exactly.
For example, PPG's DP3500 information lists different wet and dry abrasive ranges for preparing original paint or primer beneath that product. That row is not a blanket instruction for the last sanding pass before every basecoat.
Record the operation, substrate, wet/dry method, specified grade, and subsequent coating as one entry.
What changes between the two grades?
Within the same grading system and comparable abrasive construction, 500 is finer than 400. The finer option may refine a suitable preceding scratch, but it is not a shortcut for leveling a wavy panel or removing deep earlier marks.
A machine disc, a supported hand sheet, and a soft abrasive pad can produce different contact conditions. Do not treat their labels as interchangeable without the coating system's approval.
Inspect before buying more paper
- Observation
- Paper gums rapidly
- Useful next check
- Sanding readiness, temperature, loading, and product cure instructions
- Observation
- Deep isolated scratches remain
- Useful next check
- Earlier preparation and trapped contamination
- Observation
- Texture is uniform but too coarse for the topcoat
- Useful next check
- The specified final refinement stage
- Observation
- Bare metal or another layer appears
- Useful next check
- Stop and follow the repair system's spot-priming requirement
- Observation
- A low area remains while high points clear
- Useful next check
- Panel geometry and leveling, not simply a finer sheet
Use the permitted guide coat and inspection method. Remove residue and inspect in more than one light direction; slurry or dust can hide the lines you need to see.
Select the abrasive after the method
When the approved process permits hand sheets at these grades, compare 400-grit sheets in a 25-pack with 500-grit sheets in a 10-pack. Confirm the actual grading system and intended wet/dry use against the specification.
The 500-grit wet-versus-dry guide explains why the method belongs in the decision. Waterproof paper does not make an electric tool suitable for wet work.
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Release the panel by condition
Before topcoat, verify the specified scratch endpoint, cleanliness, dryness, and any time limit between preparation and coating. Check edges and repair boundaries separately. If the approved sample fails, identify the cause before changing the entire job to a finer grade. A correctly prepared panel is the target; “we used 500” is only a record of one input.
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