HumiSeal 1B31 Acrylic Conformal Coating: Application Guide and Thinning Ratios
HumiSeal 1B31 Acrylic Conformal Coating: Application Guide and Thinning Ratios
HumiSeal 1B31 is one of the most widely used acrylic conformal coatings in electronics manufacturing — and also one of the most misapplied. The coating itself is straightforward: brush or dip with Thinner 503, spray at a 1:1 ratio with Thinner 521, cure at room temperature for 24 hours handling or 30 minutes at 76°C for production throughput. Where people run into trouble is using the wrong thinner for the wrong method. This guide covers correct application by method, the right thinners for each, and the cure and inspection steps that determine whether your coating will hold up in the field.
The Full Breakdown
What 1B31 Is — and What It's Not
HumiSeal 1B31 is a single-component acrylic coating. It cures by solvent evaporation, not by chemical reaction. That means there's no mix ratio to get wrong, no pot life to worry about, and no Part A/Part B confusion. You open the can, apply it, and let it cure.
What it is: excellent moisture resistance, good dielectric properties, easy rework, and UV fluorescence for inspection. It meets MIL-I-46058C, IPC-CC-830, and IEC61086 requirements, and is compliant with REACH and RoHS Directive EU 2015/863. For most commercial and industrial electronics applications, it's the right coating.
What it's not: a high-temperature coating. The thermal shock rating is -65°C to 125°C per MIL-I-46058C, and the glass transition temperature is 14°C — meaning the coating stiffens significantly below that point. It's also not the right choice for aggressive chemical environments or applications requiring resistance to strong solvents. For those applications, polyurethane or silicone is the correct chemistry.
Application by Method
1B31 can be applied by brush, spray, or dip. Each method uses a different thinner — this is the most important thing to get right.
Brush application uses a small addition of HumiSeal Thinner 503. Apply in thin, even strokes. Don't overwork the coating; acrylic starts to skin over quickly and dragging a brush through partially cured material creates streaks that won't level out. Two thin coats are better than one thick coat. Uniformity depends on component density and operator technique.
Spray application uses HumiSeal Thinner 521 or 521EU at a 1:1 ratio by volume — equal parts 1B31 to thinner. This is a much higher thinner addition than most people expect. Spray in a well-ventilated environment so vapour and mist are carried away from the operator. The exact spray pressure will depend on your equipment and technique; adjust to achieve a uniform, pinhole-free film.
Dip coating uses HumiSeal Thinner 503 to reduce viscosity until a uniform film is achieved. Once optimum viscosity is determined, control immersion and withdrawal rate at 5-15 cm/min for even deposition. As solvent evaporates from the dip tank during production, viscosity will increase — add small amounts of Thinner 503 to maintain consistency. Check viscosity regularly with a Zahn or Ford viscosity cup.
Thinning: The Right Thinner for Each Method
Using the wrong thinner is the most common application mistake with 1B31. The TDS specifies different thinners for different methods — don't substitute.
| Method | Correct Thinner | Ratio |
|---|---|---|
| Brush | Thinner 503 | Small addition as needed |
| Spray | Thinner 521 or 521EU | 1:1 by volume (1B31 to thinner) |
| Dip | Thinner 503 | Add to achieve uniform film; adjust during production |
Don't substitute IPA, acetone, or other solvents. Substituting thinners changes the evaporation rate, which changes how the coating flows and levels, and can leave residual stress in the cured film that causes adhesion problems later.
Cure Time and Force-Cure
At room temperature, 1B31 is dry to handle in 10 minutes. Full cure is 24 hours at room temperature, but optimum coating properties — full dielectric strength, moisture resistance, and mechanical performance — take 7 days to develop fully.
For production throughput, force-cure at 76°C for 30 minutes. This is the HumiSeal-specified cure schedule. Store the product at 0-35°C and allow it to equilibrate for 24 hours at room temperature (18-32°C) before use if it's been in cold storage.
Don't power up boards before full cure. Residual solvent in an incompletely cured coating will outgas under heat, causing bubbling and delamination. That failure mode shows up in the field, not on your inspection bench.
Film Thickness and Compliance
The recommended coating thickness per the HumiSeal TDS is 25-75 microns (approximately 1-3 mils). This is the range validated for IPC-CC-830 and MIL-I-46058C compliance. Stay within this range — going significantly thicker increases the risk of cracking during thermal cycling.
UV inspection under a blacklight confirms coverage — 1B31 fluoresces under UV for ease of inspection. UV inspection is a fast and effective production QC step for identifying thin spots and missed areas.
Rework and Removal
One of the main advantages of acrylic coatings is rework. 1B31 can be removed with HumiSeal Stripper 1080 or 1080EU. Spot rework is straightforward: apply stripper to the area, allow it to soften the coating, and remove with a swab or brush. The surrounding coating is not affected if you work carefully.
For full board stripping, immersion in stripper solution is the most effective approach. Rinse thoroughly after stripping — residual stripper on the board surface will interfere with adhesion of the new coating.
Frequently Asked Questions
Can I use IPA instead of HumiSeal Thinner 503 or 521?
No. The HumiSeal TDS specifies Thinner 503 for brush and dip application and Thinner 521 or 521EU for spray. IPA has a different evaporation rate, which changes how the coating flows and levels. Use the manufacturer-specified thinner for each method.
How do I know if my 1B31 has gone bad?
Shelf life is 24 months from date of manufacture when stored properly — away from excessive heat or cold, in tightly closed containers, at 0-35°C. Signs of degraded material include increased viscosity that doesn't respond normally to thinning or skinning on the surface of the can. If you're seeing those signs, open a fresh can.
How many coats does 1B31 require?
Two coats are standard for most applications. The first coat seals the surface and establishes adhesion. The second coat builds film thickness to the target range of 25-75 microns. Allow the first coat to become tack-free before applying the second — dry to handle time is approximately 10 minutes at room temperature.
Does 1B31 require a primer?
Not for most substrates. 1B31 adheres well to FR4, most metals, and standard PCB surface finishes without a primer. The TDS notes it can be applied to substrates assembled with low-residue "no clean" materials, but recommends adequate testing to confirm compatibility with your specific assembly materials, process conditions, and cleanliness level.
What's the difference between 1B31 and 1B73?
Both are acrylic coatings, but 1B73 is a water-based formulation while 1B31 is solvent-based. Water-based coatings have lower VOC emissions and are easier to handle from a regulatory standpoint, but they have longer dry times and can be more sensitive to humidity during application. 1B31 is the more widely used formulation in production environments where throughput and proven performance are the priority.
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HumiSeal 1B31 Acrylic Conformal Coating 10.75 oz
