Before painting metal, the goal is not simply to “degrease” it, but to remove the specific contaminants that can actually destroy coating adhesion. Oil, grease, machining dust, salts, mill scale, residues from galvanising and fingerprints behave differently, so they should not be removed with the same product or the same procedure. This is why a part that looks correct immediately after painting may later start to peel, blister or develop underfilm corrosion.
If you want to select the right cleaner and avoid implementing the process by trial and error, separate the problem into three questions: what is on the surface, what metal is the part made of, and what does the subsequent painting process look like? Only then should you select the chemistry, cleaning method and cleanliness control. In practice, this stage determines whether the paint adheres to the metal or merely to the contamination left on it.
Key takeaways
- Before painting, identify the type of contamination: oil and grease, soluble salts, dust, mill scale and oxides require different removal methods.
- Solvents are mainly suitable for light organic contamination and final degreasing; they do not replace water-based cleaning when salts are suspected.
- With water-based cleaning, effective rinsing and complete drying of the surface before the next stage are essential.
- The cleaner must also be matched to the metal — carbon steel, stainless steel, aluminium and galvanised surfaces differ in their resistance to process chemicals.
- In series production, not only cleaning effectiveness matters, but also repeatability of the complete process: application method, concentration, rinsing, drying and time before painting.
- After proper surface preparation, recontamination must be limited: do not touch the part with bare hands and proceed to painting as soon as reasonably possible.
What should you use to degrease metal before painting?
The shortest answer is: use a product selected for the type of contamination and substrate, not a random solvent simply because “it evaporates quickly”. In practice, three main groups of solutions are used before painting: fast final degreasing with solvents, water-based cleaning with detergent or alkaline chemistry, and specialist process cleaners for industrial cleaning.
If the problem is typical oil and grease contamination, workshop dirt and operating residues, LOCTITE SF 7840 can be a suitable option. This product makes sense when organic contamination needs to be broken down and removed effectively without building an extensive multi-stage cleaning process. It is particularly useful in workshops, maintenance departments and when preparing individual parts or smaller batches.
If surface preparation before painting is a complete process stage rather than a quick wipe before application, Molti RS is more strongly justified. It fits better into manual, spray or immersion cleaning when oils, fats and process residues have to be removed from the entire surface rather than from a small local area. This is an important distinction. In industrial practice, many problems result from using a quick maintenance cleaner where a complete pre-paint cleaning process was required.
In series production, with repeatable parts and greater responsibility for process stability, process chemicals from BONDERITE become relevant. Selection then depends not only on the contamination but also on the application method and operating conditions. BONDERITE C-MC 1204 is used for general cleaning, BONDERITE C-MC 352 for spray cleaning of mechanical parts, and BONDERITE C-MC 3000 for heavier contamination and high-pressure cleaning.
The most important point is that the product should be selected for the problem, not the other way around. A light oil film before powder coating requires a different approach from a surface after heavy machining covered with thick grease, and a part that has been transported in winter with suspected salt contamination requires something different again. It is this third case that is most often assessed incorrectly.
Cleaning and degreasing products available in our range
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Why is “degreasing” alone often not enough?
Paint adheres properly only when it actually wets the metal surface and maintains stable contact with it. If a thin layer of oil, cleaning agent, salt or dust remains between the coating and the substrate, a weak boundary layer is created. In practice, the paint may not fail immediately, but it will have poorer adhesion, greater susceptibility to blistering and a higher risk of underfilm corrosion.
This is particularly important for process engineers and people responsible for repeatability. Adhesion problems very often do not result from the paint itself, but from an incorrectly prepared substrate. A complaint then returns to the paint shop even though the source of the problem occurred earlier: during cleaning, rinsing, drying or workstation organisation. For maintenance, this means additional rework and downtime. For purchasing, it means a greater risk of complaints, supplier disputes and costs that were difficult to avoid because the error was not in the final product but in surface preparation.
This is why degreasing should not be treated as an auxiliary task. It is part of the technology. If it is selected incorrectly, the rest of the process is built on a weak foundation.
Which contaminants really need to be distinguished before painting?
Oils, greases and coolants
This is the most common group of contaminants. They come from machining, assembly, storage or the operation of machines and parts. They are usually removed with solvents or water-based systems, depending on the scale of the process and the type of contamination. Light oils are relatively easy to remove, but solid greases, assembly pastes and heavy contamination usually require two stages: first coarse removal and then proper degreasing.
This is where the question about the “strongest cleaner” appears most often. In practice, this is the wrong question. A much more important question is whether the cleaner must deal with a thin oil film or with thick, multilayer contamination accumulated during production and operation. In the first scenario, final degreasing is often sufficient. In the second, real cleaning is required rather than simply wiping the surface.
Soluble salts
This is one of the most common causes of an incorrect decision. If chlorides or sulphates are present on the surface after transport, storage, process cleaning or operation in an aggressive environment, a solvent will not solve the problem. Salt does not behave like oil or grease, so it will not be removed effectively simply by wiping with a solvent-based degreaser.
Salts need to be washed off with water and detergent or with an appropriate water-based cleaner, followed by thorough rinsing. This is critical because an invisible salt layer can act as an electrolyte beneath the coating. The effect appears later: osmotic blistering, underfilm corrosion, delamination and rework that costs far more than the cleaning itself.
The conclusion is simple: if you suspect salt contamination, do not start with a solvent. Start with water-based cleaning with detergent and rinsing. Only afterwards should you assess whether additional final degreasing is required.
Dust, mill scale and oxides
Sanding dust, rolling mill scale and metal oxides cannot be solved with a degreaser alone. Mechanical treatment is often required, and sometimes a separate pickling or surface activation stage is necessary. If mill scale remains on steel, the paint may initially look correct, but the entire coating system will be working on an unstable substrate.
Solvent or water-based cleaning: which should you choose?
When does a solvent make sense?
Solvents work well for fast final degreasing, light organic contamination and applications where rapid evaporation matters. Isopropyl alcohol is a typical example of a product used for final surface preparation. Problems begin when solvent is used for everything: heavy grease, salts, heavily contaminated surfaces or wiping with a dirty cloth.
When is water-based cleaning the better choice?
Water-based cleaning has a major advantage when not only grease but also mixed contamination, process residues and mineral contamination have to be removed. It is also the natural choice when salts are suspected. In production systems, water-based cleaners are often safer from a process perspective and more scalable than intensive solvent cleaning.
How should the cleaner be selected for the type of metal?
| Substrate | Main risk | What to watch for when selecting | Practical direction |
|---|---|---|---|
| Carbon steel | oil, grease, salt, flash rust | a degreaser alone will not remove mill scale and rust | chemical cleaning plus mechanical treatment where necessary |
| Stainless steel | damage to passivation, contamination with free iron | avoid aggressive chemicals and contact with carbon steel | gentler degreasing and process cleanliness control |
| Aluminium | surface etching, blackening, uncontrolled reaction with chemicals | strong alkaline cleaners intended for steel may damage aluminium | neutral, inhibited or dedicated products for light metals |
| Galvanised steel | smooth surface, storage passivation, white rust | ordinary degreasing “as for steel” may not be sufficient | gentle surface preparation and verification of compatibility with the paint |
Which Melkib products are worth considering, and in which scenario?
| Scenario | Recommended direction | Example product | When it makes sense |
|---|---|---|---|
| Typical oil and grease contamination and workshop dirt | general cleaning and degreasing | LOCTITE SF 7840 | when you want to remove typical contamination quickly without an extensive process |
| Complete surface preparation before painting | manual, spray or immersion cleaning | Molti RS | when complete and more repeatable cleaning of the whole surface is required |
| General industrial cleaning | process cleaner | BONDERITE C-MC 1204 | when you need a more stable process in production conditions |
| Spray cleaning of mechanical parts | spray cleaning | BONDERITE C-MC 352 | when the process is spray-based and repeatability matters |
| Heavier contamination and high-pressure cleaning | stronger process cleaning | BONDERITE C-MC 3000 | when contamination is heavier and stronger cleaning action is required |
How should the surface be prepared step by step?
- First identify the type of contamination. Do not assume that it is only grease. Check for salts, dust, residues from previous process stages and oxides.
- Remove heavy contamination mechanically or in a preliminary cleaning step. Thick grease, assembly paste or loose deposits should not go directly into the final degreasing stage.
- Select the cleaning method. Use solvent for light contamination and final degreasing; use water-based cleaning for mixed contamination and wherever salt removal is important.
- If the process is water-based, ensure effective rinsing and complete drying. This is not a minor detail but a condition for the success of the entire operation.
- Do not touch the prepared surface with bare hands and do not place it in a dirty area. In practice, many errors occur after the cleaning itself has been performed correctly.
- If the part will be painted later, control the time between preparation and paint application. The longer the interval, the greater the risk of recontamination or flash rust.
Video: two materials from our YouTube channel about bonding metal
FAQ – what should you use to degrease a surface before painting?
Can vinegar be used to degrease metal before painting?
For a technical process, it is better not to do this. Household products do not provide predictable results and do not solve typical industrial problems such as coolants, greases, salt or machining contamination.
What is the strongest degreaser?
There is no single honest answer. The strongest product will not always be the best one. Alkaline chemistry may work well on steel, but overly aggressive cleaning can do more harm than good on aluminium or galvanised steel. A better question is: which product will be effective and safe for my metal and my process?
When should a degreaser be applied?
A degreaser is applied after heavy preliminary contamination has been removed and immediately before the next preparation stage or painting. The shorter the time between degreasing and paint application, the lower the risk of recontaminating the surface.
Do you need help selecting a metal degreaser before painting?
If the process must be repeatable or involves aluminium, galvanised steel, stainless steel, heavy contamination or suspected salts, it is worth selecting the cleaner together with the cleaning, rinsing and drying method. Tell us the type of metal, contamination, application method and subsequent painting process.
Sources
- Comprehensive analysis of degreasing and metal surface preparation processes before paint coating application: theory, technology and industrial standards.
- Product selection table – Melkib / BONDERITE / LOCTITE product materials.
- Adhesion in Paint and Coatings: Theories, Types & Test Methods.
- Surface Prep Standards – A Quick Summary – AMPP.
- Flash Rust: Causes, Formation and Prevention Methods.
- ISO 12944-4: Surface Preparation Standards.
- Avoid Plating Failures: How to Perform a Proper Water Break Test.
- Dyne test | Measuring Surface Energy.
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