Solvent Pop in Clear Coats: Tiny craters and how to avoid them

Solvent pop is a defect in clear coats that shows up as tiny craters or blisters on the surface. It happens when solvents try to escape after the coat has started to set. The look is different from dusting or orange peel and often feels uneven to the touch.

Recognize it by the small, crater-like recesses that disrupt the smooth gloss. It’s caused by trapped solvents in the film rather than a structural issue with the substrate. Use this as a diagnostic baseline when inspecting a freshly coated panel.

Basic phenomenon

Solvent pop is a common problem in clear coats where tiny craters or blisters form on the surface. This happens when solvents or gases get trapped under the paint film as it dries. As these trapped substances expand, they push against the hardening paint and eventually rupture the surface, creating small craters.

Think of it like a pimple – something’s building up underneath, putting pressure on the skin until it pops. In this case, the ‘skin’ is your clear coat, and the ‘pimple’ is the trapped solvent or gas.

The result? Tiny craters that mar the smooth finish you’re after. It’s not pretty, but understanding how it happens is the first step to preventing it.

How it differs from orange peel, fisheyes, and blushing

Solvent pop is often confused with other common finish defects. Let’s clear up the confusion:

Orange Peel: This looks like the surface of an orange – small dimples all over. It happens when the paint dries too fast or the spray pattern is incorrect. Unlike solvent pop, it doesn’t have distinct craters, and it’s usually caused by application issues rather than trapped substances.

Fisheyes: These are tiny circles that look like… well, fish eyes. They’re usually caused by contaminants in the paint or on the surface. Solvent pop craters are more random and don’t have the distinct circular shape of fisheyes.

Blushing: Blushing is a hazy, cloudy appearance that happens when moisture gets trapped under the clear coat. It’s usually caused by high humidity or damp surfaces. Unlike solvent pop, blushing doesn’t create distinct craters and often disappears once the surface dries completely.

Back to top ↑

The core cause is solvents that remain trapped inside the film as the coating cures. If they can’t escape, they push the surface outward and form a blister. This is more than a surface imperfection; it’s a failure of the film to vent internally.

Other factors include incompatible formulations or quick film formation that seals volatiles in. Mechanical factors like improper spray pattern or rapid build can also trap solvents. Always cross-check product compatibility and instructions when diagnosing symptoms.

Solvent entrapment and boil

Solvent pop happens when solvents get trapped under the clear coat. This is due to their volatility – how fast they evaporate.

Some solvents evaporate faster than others. If you mix slow and fast-evaporating solvents, the slower ones can get trapped. This is called solvent entrapment.

As these trapped solvents try to escape, they create pressure. This pressure builds up until it’s strong enough to push through the clear coat, causing tiny craters on the surface – that’s your solvent pop.

Intercoat compatibility and polymer crosslinking

Incompatible layers can also cause solvent pop. If you apply a clear coat over a layer that’s not fully cured, the solvents in the clear coat can get trapped.

Polymer crosslinking is when the molecules in the paint or clear coat link together to form a solid film. If this process is interrupted – maybe by applying another coat too soon – solvents can’t escape and get trapped instead.

These trapped solvents then try to escape, creating pressure that leads to solvent pop.

Substrate and trapped contaminants

The surface you’re painting on – the substrate – can also contribute to solvent pop. If it’s porous, solvents can seep in and get trapped.

Residues or moisture on the substrate can also cause problems. These can prevent solvents from evaporating properly, leading to entrapment and eventually, solvent pop.

That’s why it’s crucial to prepare your surface properly before painting. Make sure it’s clean, dry, and free of any contaminants that could trap solvents.

Back to top ↑

Temperature and humidity levels influence how fast solvents evaporate and how the film sets. High or fluctuating temperatures can drive rapid evaporation and pressure build-up. Humidity can affect film formation and solvent release dynamics.

Surface temperature and the materials used (solvent content, additives, or primers) matter as well. Always verify product data sheets and manufacturer guidelines before proceeding with a job. Look for any notes on venting or cure windows and stay within recommended ranges.

Temperature and humidity effects

Solvent pop can happen when temps are too cold or hot, or change rapidly. Here’s why:

Cold conditions slow down solvent evaporation. Solvents can’t escape fast enough as the clear coat dries, leading to tiny craters.

Hot conditions speed up evaporation. The top layer dries too quickly, trapping solvents underneath. When these trapped solvents finally try to escape, they cause pop.

Rapid temp changes can also create stress in the film, pushing solvents out and causing pop.

Solvent blends and VOC content

Different solvents have different boiling points. When you mix them, lower-boiling solvents evaporate faster than higher-boiling ones. This can cause internal solvent migration:

Fast-evaporating solvents leave behind slower ones. As these slow-evaporators try to escape, they push through the now-dry top layer, causing pop.

High VOC content (Volatile Organic Compounds) also increases risk. More solvents mean more chance of entrapment and pop.

Check your clear coat’s data sheet for its solvent blend and VOC content. Lower VOCs and balanced solvent blends reduce the risk of solvent pop.

Substrates, primers, and old finishes

Certain substrates and primers can increase solvent pop risk:

Soft or solvent-sensitive layers can absorb solvents from your clear coat. As these absorbed solvents try to escape, they push through the film, causing pop.

Old finishes might have trapped contaminants or incompatible chemicals. These can react with your new clear coat, leading to solvent pop.

Always test for compatibility before applying a clear coat. Use a small, hidden area first. If no issues arise after 24 hours, you’re good to go.

Back to top ↑

Start with thorough cleaning and degreasing to remove oils and contaminants. Ensure the substrate is dry and free from residues that can trap moisture. Allow adequate time for any cleaners to flash off as per product instructions.

Condition the substrate and verify compatibility with the clear coat. Follow recommended flash times and compatibility checks rather than guessing. Double-check label directions and datasheets before coating.

Surface cleaning and contaminant removal

Before applying any clear coat, it’s crucial to ensure your surface is clean and free of contaminants. Here’s how:

Wax, silicone, or even moisture can cause solvent pop. Use a suitable cleaner to remove these.

Always verify your cleaning job by taping off a small area and applying clear coat. If it pops, clean again.

Primer and basecoat selection and cure

Choosing the right primer and basecoat is vital. Follow these steps:

Check manufacturer guidelines for intercoat compatibility. Some clear coats don’t play nice with certain primers.

Allow your primer and basecoat to fully cure before applying the clear coat. This could take days or weeks, depending on the product.

Respect the manufacturer’s specified recoat window. Applying too soon can lead to solvent pop.

Environmental control and staging

Temperature and humidity play a big role in solvent pop. Here’s how to manage them:

Stabilize the temperature in your spray area as per product guidelines. Too hot or too cold can cause issues.

Keep an eye on humidity. High humidity can trap moisture and lead to popping. Use dehumidifiers if needed.

Warm or cool panels as required, following manufacturer instructions. This helps ensure even curing and reduces the risk of solvent pop.

Back to top ↑

Set up the spray gun for consistent atomization and stable spray patterns. Maintain proper nozzle size and air pressure as specified by the product instructions. Keep a consistent distance and motion to avoid uneven solvent release.

Overlap passes evenly and follow recommended pass counts. Do not push or rush layers; allow for appropriate flash between passes if instructed by the manufacturer. Adhere strictly to stated spray parameters in the product data sheet.

Gun selection and setup

Choosing the right spray gun is crucial to prevent solvent pop. Start by selecting a gun with adjustable needle and nozzle sizes. A smaller tip can help create a finer atomization, reducing the chance of solvent entrapment.

Adjust fluid pressure according to your material’s viscosity. Too high or low pressure can lead to improper film formation. Check your manufacturer’s guidelines for recommended pressure ranges.

Set your spray pattern to ‘fan’ or ‘flat’ for even coverage and reduced solvent pooling. This helps ensure a consistent, smooth finish with minimal risk of solvent pop.

Wet film thickness and layering strategy

Applying coats in thin, incremental layers allows solvents to evaporate gradually, reducing the likelihood of entrapment. This technique is often referred to as ‘building’ your finish.

Instead of applying heavy wet coats, aim for a wet film thickness within manufacturer recommendations. This usually ranges between 1.5 to 3 mils (0.0015 to 0.003 inches).

Check your product’s technical data sheet for specific guidelines on layering strategy and wet film thickness limits.

Flash times between coats

Allowing adequate flash time between coats is vital to prevent solvent pop. This gives solvents enough time to evaporate before applying the next layer.

Always follow the manufacturer’s specified flash times. If environmental factors like humidity or low temperature slow drying, err on the side of caution and extend your flash times.

To check if a coat is dry enough for the next application, lightly touch it with a gloved finger. If no tackiness remains, it’s ready for the next layer.

Back to top ↑

Control panel and workshop temperatures to support a gradual solvent evaporation. Avoid sudden temperature changes that can trap volatiles. Manage air movement to prevent dust while still ventilating the area as needed.

Handle panels gently during initial cure and avoid stacking or reworking too soon. Proper handling helps solvent escape without disturbing the forming film. Always align with the cure guidance from the manufacturer’s instructions.

Accelerated vs ambient cure considerations

Curing clear coats involves driving out solvents. You can speed this up with accelerated methods like oven baking or infrared (IR) heat, but be careful.

Oven bake: Heat helps drive off volatiles, but too much too fast can cause solvent pop if there’s trapped material. Keep temps around 140-160°F (60-71°C).

IR heat: This is gentler than ovens. It heats the surface, not the air. But again, watch for rapid heating causing solvent pop.

Air cure is slower but safer if you’re unsure. It gives solvents time to escape naturally without risking rapid expansion.

When to delay polishing or buffing

Polishing too soon can make trapped defects worse. Here’s why:

Compound and polish: These processes apply pressure and heat, which can force trapped solvent out, causing pop.

Wait until the coat is fully cured before compounding or polishing. This usually takes 7-14 days at room temp, longer if accelerated curing was used. Check with your product’s specific cure time guidelines.

Back to top ↑

Use a simple visual checklist to catch early signs of solvent pop on test panels. Look for crater-like features, dull spots, or surface texturing that differs from surrounding gloss. Compare against known good samples if available.

Prepare test panels or swatches to verify results before committing to a full job. Note any discrepancies and consult the product label or data sheet for guidance on corrective actions.

Conservative options start with spot sanding and localized recoat when the defect is limited. Feather the edges to blend the repair with the surrounding area. Follow proper surface prep before applying additional coats.

For deeper or widespread defects, consider full-panel stripping and reapplication. Decision factors include the severity, depth of the pop, and the feasibility of a repair within your project constraints. Verify repair procedures against manufacturer instructions and local rules.

Spot repair and localized sanding

For minor solvent pops, spot wet-sanding is your best bet. Here’s how:

First, lightly wet-sand the affected area using 1200-grit or finer sandpaper. Wet sanding helps prevent dust and keeps the clear coat cool.

Next, feather back to sound edges using a decreasing grit sequence (e.g., 800, 600, 400). This ensures a smooth transition from the repaired area to the surrounding surface.

Tip: Be patient and take your time. Rushing can cause more damage or leave visible sanding marks.

Full-panel recoat or strip-and-repaint

If solvent pops cover a large area, consider a full panel recoat or stripping and repainting. Here’s when to choose each:

Opt for a full panel recoat if the clear coat is still in good condition elsewhere and the pops are isolated to one section. Lightly sand, clean, and recoat the entire panel.

For extensive damage or multiple panels affected, it’s better to strip and repaint. Start by stripping the old clear coat using a suitable stripper. Follow up with thorough cleaning, sanding, and priming before applying new clear coats.

Note: Always ensure proper ventilation when working with chemicals like strippers and thinners.

Polishing and cosmetic masking solutions

For shallow solvent pops, polishing can sometimes hide the imperfections. Here’s what to consider:

If the craters are minor (<1/64″ deep) and don’t affect the structural integrity of the clear coat, polishing might be enough. Use a rubbing compound or polish designed for your clear coat type.

However, if the pops are deeper or numerous, polishing won’t fix the problem. In these cases, you’ll need to structurally repair the area first, either by spot sanding (as mentioned earlier) or stripping and repainting.

Tip: Before polishing, test a small, hidden area to ensure it doesn’t cause further damage or discoloration.

Back to top ↑