Valve seats may be conical or flat. Either way, grinding is normally done in stages, because each stage has a different job: one removes damage, the next refines the surface, and the last brings the sealing surface to the required condition.

The abrasive used at each stage serves a different purpose. Coarser abrasive is used to remove grooves, pits and scratches, while finer abrasive is used to remove minor scratches and further refine the sealing surface.

1. Rough grinding — remove the damage

Grooves, pits and scratches have to come off first. For portable valve grinding machines, rough grinding typically uses approximately 80-grit abrasive material. For stationary valve grinding machines, the rough-grinding abrasive may range from 60 to 120 grit, depending on the surface condition. The goal here is not a fine finish; it is to prepare the sealing surface for the next stage.

Starting with fine grinding may leave deeper grooves, pits or scratches insufficiently addressed, because rough grinding is the stage intended to remove these defects.

2. Fine grinding — remove minor scratches

Once the damage is gone, fine grinding uses approximately 240-grit abrasive material to remove minor scratches left by rough grinding, and improves the sealing-surface condition. The scratch pattern left behind is finer, and the surface starts to approach the condition it needs for sealing.

Moving on to polishing while coarse marks remain may leave the sealing surface insufficiently prepared. Polishing is intended to further refine the surface after the earlier grinding stages.

3. Polishing — finish to the required surface condition

The last stage uses 400-grit or finer abrasive material, according to the required surface condition. It further refines the sealing surface to the finish required.

Polishing is a finishing step, not a repair step: it works on a surface the earlier stages have already prepared, and polishing alone does not decide whether a valve seals.

Why the order matters

The three stages form a progressive sequence. Rough grinding removes grooves, pits and scratches. Fine grinding removes minor scratches and further improves the surface condition. Polishing then refines the sealing surface according to the required finish. Following the correct sequence helps prepare the surface step by step.

Two practical notes. First, the machine still has to fit the job before any of this starts: measure the actual sealing-face diameter, the required working depth and the installation space (see our selection guide, Choosing a Valve Grinding Machine: Three Checks Beyond DN). Even when the grinding process is followed correctly, the machine and tooling must be suitable for the workpiece and operating conditions. Second, keep the work clean between stages, so abrasive from one stage is not carried into the next.