Vibratory Finishing Compounds and Additives Explained
The compound is the chemistry that makes vibratory finishing work. It cleans the parts, inhibits corrosion, lubricates the media, and controls whether you get a cut, a burnish or a polish. This guide covers the main compound families and when to use additives on top.
What the liquid compound does
Cleans: keeps oil, coolant and swarf moving out of the machine rather than embedding into the media.
Inhibits: prevents rust on ferrous parts while they are wet in the machine and for a short period after unloading.
Lubricates: modulates how aggressively the media acts. More compound = more lubrication = less cutting, more polishing.
Buffers: keeps pH in a working range that neither attacks aluminium nor lets steel flash-rust.
Ferrous vs non-ferrous compound
A rust-inhibiting compound suitable for ferrous work is used on mild steel, alloy steel, and cast iron. It contains corrosion inhibitors that keep parts rust-free through the cycle and leave a light protective film on unload.
A general-purpose compound suitable for non-ferrous metals is used on aluminium, brass, copper, zinc, titanium, and, in practice, on stainless steel. Stainless is chemically ferrous but corrosion-resistant, so it is treated as non-ferrous in most subcontract shops. This type of compound keeps the surface bright and adds no inhibitor residue that would interfere with plating.
The wrong compound is a common cause of quality issues: ferrous inhibitor on aluminium leaves haze; non-ferrous on steel flash-rusts within hours.
Citric acid boost for rusty mild steel
Steel parts that arrive already rusted from storage need help beyond the standard compound. Adding citric acid at 0.5 to 1% at the start of the cycle strips light surface rust in the first 20 to 30 minutes.
The citric is consumed by the reaction and neutralises before the cycle ends, so parts come out cleanly inhibited by the normal ferrous compound. This avoids the operator having to run a separate pickle step.
For heavier rust or scale, run a dedicated descale stage with fresh media before the deburr stage.
Degreasant powder for heavy soiling
Parts coming straight off a CNC with cutting oil benefit from a degreasant powder boost in the first 10 to 15 minutes. This lifts the oil into the flow-through water so the media does not glaze.
Do not run degreasant continuously, it depletes the corrosion inhibitor and, on aluminium, can etch high-silicon alloys.
Paste polish and dry stages
Superfinishing paste replaces liquid compound for the final polish. A standard paste with porcelain media in a barrel or tub gives a mirror finish on steel; a stainless-specific paste variant works on 304 and 316.
Paste polish is dry, no water flow, and runs long, often overnight. Media and paste are reused across many cycles.
Maize (corncob) carrier is also used dry, either for polishing or for drying parts post-wet-process.
Water flow rate as a control lever
On a ~300 L bowl the standard flow rate is around 1 L/min. This is a starting point, treat it as a control lever.
Reduce flow (0.3 to 0.5 L/min) for aggressive cutting.
Increase flow (1.5 to 3 L/min) for polishing or burnishing.
Metered dosing (30 to 35 L total per cycle, no continuous flow) is used for controlled-chemistry work where the compound must not be diluted.
Dry (no water, paste or maize only) for final polish and drying.
Frequently asked questions
Can I use the same compound for stainless and mild steel?
Yes in a mixed shop, but non-ferrous compound gives better stainless results. Use ferrous compound only if mild steel dominates the mix.
How much compound per cycle?
Typically 1 to 3% concentration in the flow-through water. Follow the supplier's data sheet, over-dosing does not accelerate the process, it just costs more.
Do I need to inhibit stainless steel?
No. Stainless does not need inhibitor and will not flash-rust after unload.