Before you start any repair work on a scratched control valve spool in an excavator, you need to run a full inspection to map out the exact damage. You should first take the entire valve assembly apart and rinse every part with a non-residue cleaning fluid to strip away all hydraulic oil, dirt and metal shavings that might hide small scratches. Use a micrometer to measure the outer diameter of the spool at multiple points along its length, and check the matching valve bore with an internal dial gauge to record the current fit clearance. Most healthy spool and bore pairs keep their clearance between 0.01 mm and 0.025 mm, so any gap far beyond that range means the wear has gone past minor levels. You also need to run a gloved finger along the spool surface to feel for raised burrs or deep axial grooves, and mark every damaged spot with a soft marker so you will not miss them in later steps.
Prep the scratched spool surface to make it ready for follow-up repair work. Use a fine flat file to knock off all sharp burrs along the edges of the scratch, so no loose metal bits will cut into the sealing surface later. For shallow scratches that sit under 0.5 mm deep, you can use a small sharp chisel to widen the scratch slightly, then grind the area with coarse emery cloth or a light angle grinder to expose a fully clean bare metal surface. For deeper grooves that run over 1 mm, you need to carve out a shallow trough along the full length of the scratch, making sure no residual fatigue layer sticks to the base metal. Once the rough grinding is done, wipe the whole spool down with a metal cleaner to get rid of all oil traces, fine dust and grinding debris, and set it in a dry, dust-free spot to air dry completely before you move on.
Fill and reshape the damaged area to bring the spool back to its original size. Mix the wear-resistant repair compound at the exact ratio that fits the on-site working conditions, and spread the first thin layer evenly over the pre-treated scratch from top to bottom, pressing hard to push out every trapped air bubble. Let this first layer sit for around 1 hour until it turns semi-cured, then mix a fresh batch of the compound and apply the second coat, making the new material sit 1 mm to 2 mm higher than the original spool surface. After the second coat cures naturally for 1 hour, use an infrared heat lamp to warm the whole spool steadily for 3 hours, so the repair material can harden fully and form a tight bond with the base metal. Do not touch or move the spool during the heating process, as sudden shifts or temperature drops might create tiny cracks inside the filled area.
Do precision finishing to get the spool back to the required tolerance. Use a soft file and fine scraper to shave down the excess repair material slowly, and stop often to measure the outer diameter so you never grind below the original spool size, leaving a 0.5 mm margin for final fine finishing. When you get close to the target size, wrap 80-grit emery cloth around a custom-made grinding sleeve, and slide it along the spool surface to polish the repaired area evenly. Once the repaired spot sits 0.2 mm above the original spool dimension, switch to fine diamond abrasive paste, and keep grinding until the outer diameter matches the size that forms a proper fit with the restored valve bore. Every few minutes during this step, wipe the spool clean and test its fit in the valve bore with a thin layer of clean hydraulic oil, to make sure the sliding motion stays smooth with no stuck spots or uneven resistance.
Run post-repair checks and on-site protection to lock in the repair quality. Wipe the fully finished spool and the matching valve bore one more time with clean hydraulic oil to wash away all leftover fine grinding particles. Slide the spool into the valve bore slowly, and move it back and forth the full length of the bore multiple times, to feel for even resistance and confirm no tight points show up at any position. If you spot tiny pinholes or small gaps on the repaired surface after the test, you can re-apply a tiny amount of repair compound to fill the flaw, then do a light round of fine grinding again. Keep the whole work area free of floating dust for the full curing period, so no outside impurities stick to the fresh repaired surface and ruin the precision fit you just worked to set up.