Media does the mechanical work in a vibratory bowl; the compound manages everything else that determines whether parts come out clean and consistent. A properly chosen compound performs several jobs at once:

1. Keeps the media cutting. Compounds continuously scrub metal fines and abraded media particles off the media surface. Without that cleaning action the media glazes and its cut rate collapses.
2. Suspends and removes soils. Oils, dirt, and swarf from the parts are emulsified and carried out with the process water instead of redepositing on the work.
3. Controls the chemistry. Buffered pH protects the metal being finished — critical for aluminium and zinc — while wetting agents ensure fresh solution reaches every part surface.
4. Conditions the surface. Depending on formulation, the compound cleans, brightens, degreases, descales, or burnishes as the media works.
5. Protects afterwards. Rust inhibitors in the compound leave a short-term protective film on ferrous parts as they leave the bowl.

Running media with plain water — or worse, dry — is the most common cause of dirty, dull, inconsistently finished parts.

They sit at opposite ends of the finishing process and are formulated differently.

Deburring (cutting) compounds support aggressive material removal. Used with abrasive ceramic or plastic media, they are typically mildly alkaline cleaners that keep the media face sharp, flush heavy metal fines away, and stop the swarf from rolling into the part surface. The goal is fast, clean stock removal — edge breaking, flash removal, scale stripping — with the compound preventing the debris of that work from spoiling it.

Burnishing compounds remove no material at all. Used with non-abrasive media — hardened steel balls and shapes, or porcelain — they are soap-based formulations that lubricate the contact between media and part, allowing the media to planish and compress the surface to a bright, reflective finish. Burnishing soaps also keep steel media itself clean and bright, which the mirror finish depends on.

Many shops run both in sequence: an abrasive cut with a deburring compound, then a burnishing cycle for final brightness. The compounds are not interchangeable — a cutting compound in a burnishing load dulls the finish, and soap in a cutting load slows removal.

Both chemistries finish parts well; the choice is mostly about process control and equipment.

Liquid compounds are the standard for flow-through vibratory systems. Metered continuously with fresh water by a dosing pump, they hold the bowl at a constant, repeatable concentration, which keeps finish quality identical from the first part to the last. Effluent stays consistent for treatment, and there is no dust. Their advantages fade only where there is no dosing equipment.

Powder compounds suit batch operation — a measured scoop per load in a closed or recirculating bowl, common in smaller shops, rotary barrels, and hobby tumblers. They ship and store economically and can carry higher loadings of certain actives. The trade-offs: concentration drifts within the cycle as the compound depletes, dissolving must be complete to avoid paste pockets under the media, and handling creates dust.

As a rule of thumb: continuous or high-volume finishing favours liquid with automatic dosing; occasional batch work favours powder for its simplicity. Follow the media manufacturer's compatibility guidance either way, since compound and media are engineered as a system.

Dose by the manufacturer's ratio for your machine size, then adjust from what the parts and solution tell you.

Batch (closed) operation: typical loadings run around 15–30 g of powder, or 10–30 ml of liquid concentrate, per 10 L of bowl capacity, dissolved in enough water to reach the recommended level — usually just visible at the top of the media mass as it rolls. Overfilling with water cushions the media and kills the cutting action; too little lets soils concentrate and redeposit.

Flow-through operation: liquid compound is metered continuously, commonly at 1–5% concentration in the fresh water feed, with the flow rate set so the bowl fully exchanges its solution every one to two hours.

Read the process as it runs. Foam climbing over the media, slippery parts, and slowed cutting mean too much compound; grey water, tide marks, dull or smeared parts, and media that looks dirty mean too little or exhausted solution. In batch work, replace the solution rather than topping it up once it darkens — spent compound full of suspended fines is itself an abrasive contaminant.

Freshly finished steel is chemically clean bare metal — it will flash-rust within hours in humid air unless protected. Two compound strategies prevent that:

1. Inhibited process compounds. Many deburring and burnishing compounds are formulated with rust inhibitors built in, so parts leave the bowl already carrying a short-term protective film. If ferrous work is your staple, run one of these as the standard process compound.
2. A dedicated inhibitor rinse. After finishing, parts get a short cycle — in the bowl or a separate tank — in a rust-inhibiting rinse concentrate. This displaces the process water and deposits a clear amine- or nitrite-free barrier film. It is the more controllable route when the same machine also runs non-ferrous work.

Either way the protection is temporary — typically days to a few weeks of indoor storage — designed to bridge the gap to oiling, coating, plating, or assembly, not to replace it. Dry parts promptly and thoroughly; trapped rinse water defeats any inhibitor.

One caution: match the inhibitor to downstream processes. Some films must be cleaned off before painting or plating, so confirm compatibility if parts go on to a coating line.

General-purpose compounds exist and work acceptably across metals for undemanding jobs, but metal-specific chemistry earns its keep the moment finish quality or part value matters.

The core issue is pH and inhibitor chemistry. Steel tolerates — and cleans best in — alkaline compounds, and needs rust inhibition. Aluminium is amphoteric: strongly alkaline solutions etch, darken, and can pit it, so aluminium compounds are formulated near-neutral or specifically inhibited against attack. Brass and copper alloys stain and tarnish in the wrong chemistry and shine best with dedicated brightening agents; zinc die castings are more sensitive still.

A true multi-metal compound is deliberately mild — near-neutral pH, broad inhibitor package — which is exactly why it cleans and cuts more slowly than a targeted product. Running mixed loads of dissimilar metals together carries a further risk the compound cannot fix: galvanic staining and cross-contamination, such as steel fines embedding in soft aluminium surfaces.

Practical guidance: sort work by metal where possible, keep a steel compound and a non-ferrous compound on the shelf, and reserve the general-purpose product for genuinely mixed, non-critical batches.

Work through the usual suspects in order:

1. Exhausted or under-dosed compound. The most common cause. Spent solution loaded with fines redeposits soil as a grey film. Dump, rinse the load, and recharge at the correct dose — do not just add more concentrate to dirty water.
2. Not enough water flow or volume. Soils concentrate instead of leaving. In flow-through bowls check the dosing pump and drain screen; in batch bowls check the fill level.
3. Glazed or contaminated media. Media that looks shiny or feels greasy has stopped cutting and is smearing soil around. Run a media-cleaning cycle with a descaling/cleaning compound, or replace worn-out media.
4. Wrong compound for the metal. Alkaline steel compounds dull aluminium and stain brass; burnishing soap in a cutting cycle leaves a smeared, incomplete finish. Match chemistry to metal and process stage.
5. Overloaded bowl or part-on-part contact. Too many parts, or delicate parts without enough media separation, mar each other faster than the media can finish them.
6. Foam. Excess compound or oily parts generate foam that cushions the media action — parts emerge under-finished. Reduce dose, pre-degrease oily work, or add defoamer.

Fix in that order — solution first, media second, chemistry third — and re-run a small test load before committing full batches.