Brush phosphating is the most accessible method of pretreatment for powder coaters. It requires minimal equipment, no tank setup, and can be done on parts of any size — from small brackets to oversized structural steel. This guide walks you through the complete process using InstaPhos 3-in-1 Excel.
What You Will Need
Before you start, gather the following:
- InstaPhos 3-in-1 Excel solution
- Clean water (tap water is fine)
- A non-metallic container — plastic bucket, HDPE tray, or fibreglass tub (never use metal containers as the solution will react with them)
- Polyurethane foam brush or sponge — do not use metallic brushes or wire wool
- Measuring cup for mixing ratios
- Clean cotton rags or wipes
- Personal protective equipment — rubber or nitrile gloves, safety glasses
Optional but helpful:
- A plastic spray bottle for application on hard-to-reach areas
- A well-ventilated workspace or outdoor area
- Plastic sheeting to protect your floor
Step 1: Prepare Your Workspace
Set up in a well-ventilated area. Phosphating solutions contain mild acids, and while InstaPhos 3-in-1 Excel is formulated to be low-fuming, good ventilation is always a sensible precaution. Lay down plastic sheeting if you are working on a floor you want to protect.
Make sure the parts you plan to treat are within reach and you have enough solution mixed for the job. A good rule of thumb is approximately 80 to 100 ml of mixed solution per square metre of surface area.
Step 2: Mix the Solution
Pour InstaPhos 3-in-1 Excel into your non-metallic container and add an equal volume of clean water. The standard mixing ratio is 1:1 (one part InstaPhos to one part water).
For heavily rusted or heavily soiled parts, you can use a stronger concentration — up to neat (undiluted) solution. For lightly soiled, new steel with minimal oil, a 1:2 dilution (one part InstaPhos to two parts water) may be sufficient.
Stir gently with a plastic stirrer to ensure a uniform mix. The solution should be used at room temperature (25 to 40 degrees Celsius). No heating is required.
Step 3: Inspect and Pre-Clean the Part
Before applying the phosphating solution, inspect the metal surface:
- Heavy mud, dirt, or debris: Remove mechanically with a brush or wipe. The 3-in-1 solution handles oils and light rust, but it is not designed to remove caked-on dirt.
- Heavy weld spatter: Grind or chip off before treatment. The solution cannot remove raised weld spatter.
- Mill scale on hot-rolled steel: If the scale is thick and tightly adherent, mechanical removal (grinding or blasting) may be needed first. The 3-in-1 solution can handle light mill scale but will struggle with heavy, intact scale.
For most fabricated parts that have been through cutting, welding, and handling, the surface will have shop oils, light rust, and fingerprints — all of which the 3-in-1 solution handles well.
Step 4: Apply the Solution
Dip your polyurethane brush or sponge into the mixed solution and apply it liberally to the metal surface. Work in overlapping strokes to ensure complete coverage. The solution should wet the entire surface — do not leave dry spots.
Key tips for application:
- Apply generously — a thin film is not enough. The surface should glisten with solution.
- Work the solution into corners, weld joints, and textured areas. These are the spots most likely to be missed and most likely to cause coating failure later.
- Re-dip your brush frequently. The solution is consumed as it reacts with the metal, so a brush that has gone dry is no longer treating the surface.
- For vertical surfaces, work from bottom to top to ensure solution does not run off before it has time to react.
- For large parts, work in sections to ensure no area dries before being properly treated.
You will notice the solution reacting with the metal surface. Rust dissolves, oils emulsify, and the clean metal begins to take on a slightly darker, grey-blue tone — this is the phosphate conversion coating forming.
Step 5: Allow the Coating to Develop
After applying the solution, allow the part to air dry naturally. Do not rinse. The drying period allows the phosphate conversion coating to fully develop and set. This typically takes 30 to 45 minutes at room temperature, depending on ambient conditions.
In humid conditions or during the monsoon season, drying may take longer. You can speed it up with a fan, but avoid using direct heat (heat guns or ovens) at this stage as it can disrupt the coating formation.
The finished surface should have a uniform grey to dark grey appearance with no visible rust, oil, or bare metal patches. If you spot any areas that still show rust or appear untreated, apply a second coat of solution to those spots and allow to dry again.
Step 6: Proceed to Powder Coating
Once the phosphated surface is completely dry, it is ready for powder coating. There is no need for a separate rinse step — this is one of the key advantages of the 3-in-1 process.
Apply your powder coating as normal. The phosphated surface provides excellent adhesion for both epoxy and polyester powder coatings. You will notice that the powder lays down more evenly on a phosphated surface compared to bare metal.
Important: Do not leave phosphated parts exposed for extended periods before coating. While the phosphate layer provides temporary corrosion protection, it is designed as a pretreatment for coating, not a standalone finish. Ideally, powder coat within 24 hours of phosphating. If parts must wait longer, store them in a dry, covered area.
Coverage Rates and Consumption
With the standard 1:1 mixing ratio, expect approximately:
- 80 to 100 ml of mixed solution per square metre for lightly soiled, new steel
- 120 to 150 ml of mixed solution per square metre for moderately rusted or oily parts
- 150 to 200 ml of mixed solution per square metre for heavily rusted parts requiring multiple applications
A 5-litre can of InstaPhos 3-in-1 Excel, mixed 1:1 with water, gives you 10 litres of working solution — enough to treat approximately 65 to 125 square metres of metal surface, depending on the condition of the parts.
Troubleshooting Common Issues
The surface still shows rust after treatment: Apply a second, heavier coat. For heavily rusted parts, consider using the solution at neat (undiluted) concentration or allow more contact time before the first coat dries.
The coating appears patchy or uneven: This usually means the solution was applied too thinly or dried too quickly. Re-apply to the affected areas. Ensure you are using enough solution per square metre.
The solution is not reacting at all: Check that your mixing ratio is correct and that the solution has not exceeded its shelf life. Also ensure the surface is not coated with a heavy layer of paint, zinc plating, or galvanizing — the 3-in-1 solution is designed for bare iron and steel.
White powdery residue on the surface: This can occur if the solution is applied too thickly in one area. Wipe off the excess with a clean dry cloth before proceeding to powder coating.
Brush phosphating with a 3-in-1 solution is one of the simplest and most effective improvements you can make to your powder coating process. It takes minimal time, requires almost no equipment, and the quality improvement in your finished work will be immediately visible.