Why paint swells when spray painting a car: analysis of reasons and solutions

Introduction: the problem that every second beginner faces

Spray painting a car seems like a simple solution for local body repairs - no expensive equipment, special skills or a visit to a car service are required. However, even with careful preparation, many are faced with an unpleasant surprise: fresh paint begins to swell, forming bubbles or "craters" on the surface. This defect not only spoils the appearance, but also negates all efforts to restore the paintwork.

According to surveys among car owners engaged in self-repair, more than 60% of cases of paint swelling are associated with errors at the surface preparation stage or violation of application technology. At the same time, 30% of problems arise due to poor quality materials, and the remaining 10% are due to external factors such as humidity or temperature. In this article, we will analyze all the possible reasons why paint “boils” when painting with a spray can, and we will give clear recommendations on how to avoid this.

1. Improper surface preparation is the main cause of defects

The main culprit behind paint blistering is poor adhesion (adhesion) of the new layer to the base. If the surface is not free of dirt, grease, or old peeling paint, solvents from the can will react with these contaminants, producing gases. These gases lead to the appearance of bubbles.

What exactly needs to be done before painting:

• 🧽 Degreasing: use specialized tools, e.g. APP W900 or Body 700. Regular white spirit or acetone leaves a film that impairs adhesion.
• 🔍 Stripping: Remove rust and old paint to bare metal (or a durable layer) using sandpaper P80–P180.
• 🧴 Primer: apply epoxy primer (for example, Novol Protect 360) - it creates a sealed layer that prevents reaction with the metal.
• 🌡️ Drying: after each stage, keep the part for at least 30 minutes at temperature 20–25°C.

Pay special attention transitions between old and new coating. If you do not make a “smooth” cut of the old paint (the so-called “feder”), the new layer will come off along the border. To do this, use an abrasive P500–P600 and matting paste.

2. The influence of humidity and temperature: why the paint “boils”

Spray paint cans contain solvents that evaporate during application. If the air humidity exceeds 60% or temperature is lower +15°C, the evaporation process is disrupted. As a result:

• 💧 Moisture from the air condenses on the surface, forming microbubbles.
• ❄️ At low temperatures, the solvent does not have time to evaporate evenly, creating “craters”.
• ☀️ When overheating (above +30°C) the paint dries too quickly, without having time to “spread,” which also leads to defects.

Optimal conditions for spray painting:

Parameter	Minimum value	Maximum value	Ideal value
Air temperature	+15°C	+25°C	+20°C
Surface temperature	+18°C	+28°C	+22°C
Humidity	30%	60%	40–50%
Wind speed (for outdoor painting)	0 m/s	2 m/s	0–1 m/s

If painting takes place in the garage in winter, use infrared heaters (for example, Ballu BIH-LW-3.0) for local heating of the part. It is important to avoid direct heating — place the heater at a distance of at least 1.5 m.

3. Low-quality materials: how to choose the “right” canister

Cheap paints in cans often contain aggressive solvents (such as xylene or toluene), which react with soil or metal, releasing gases. In addition, they may not have enough plasticizers, causing the coating to become brittle and prone to swelling.

What to look for when choosing:

• 🏷️ Composition: give preference to colors on acrylic or urethane basis (for example, Mobihel or Motip). They are less sensitive to humidity.
• 📅 Expiration date: Solvents in expired cans lose their properties, which leads to uneven drying.
• 🔬 Compatibility: If painting over old paintwork, check that the new coat is compatible with it (indicated on the label).
• 🎨 Pigmentation: Paints that are too cheap contain little pigment and require multiple applications, which increases the risk of defects.

One of the most reliable options is paints in cans with a two-component system (for example, SprayMax 2K). They require mixing before use but provide professional quality with minimal risk of blistering.

How to check the quality of paint before purchasing?

1. Shake the can - if you hear only a liquid “splash” without “balls”, this is a sign of delamination.

2. Press the nozzle upside down: if the paint comes out evenly, without splashing, the composition is good.

3. Look at the production date: the optimal age of the can is no older than 12 months.

4. Application technique: mistakes that spoil the result

Even with perfect preparation and quality materials, paint can swell if damaged. spray technique. Typical mistakes:

• 🌀 Too close distance: holding the can closer 20–25 cm, you create a “wet” layer that does not have time to ventilate.
• ⏳ Thick Layers: One pass should cover the surface 50–70%. Full coverage is achieved in 2-3 thin coats.
• 🔄 Irregular movements: move the can parallel to the surface, without zigzags. Movement speed - 30–40 cm/s.
• 🛑 Interrupting a Layer: if you stop in the middle, a “roller” of paint will form at the stop point, which will later swell.

Correct application sequence:

1. Apply the first layer ("developing") - it should be translucent.
2. Let dry 10–15 minutes (until matte).
3. Apply a second coat, overlapping the first by 50%.
4. Repeat drying and apply final coat.

Cleaning the surface from dirt and grease|Sanding with sandpaper P180–P240|Degreasing with a special compound|Applying primer (if necessary)|Drying each layer for at least 15 minutes|Checking the temperature and humidity in the room-->

If the paint starts to bubble immediately after application, do not try to “paint over” the defect - this will only make the problem worse. It is better to remove the layer with solvent and start again.

5. Reaction with previous layers: chemical incompatibility

One of the most insidious causes of bloating is chemical conflict between old and new paint. This happens if:

• 🔥 Left on the old surface nitro paints (they contain aggressive solvents that “raise” the new layer).
• 🧪 New paint on acrylic base is applied on top alkyd without insulating layer.
• 🛢️ Use primer or putty based on polyester, incompatible with paint.

How to avoid a reaction:

1. If you don't know the type of old paint, apply insulating soil (for example, Novol Protect 340).
2. To test compatibility, apply paint to a small area and wait 24 hours.
3. When working with metallics or mother-of-pearl use base layer the same manufacturer.

Particular care must be taken when working with restoration cans (for example, Rust-Oleum). They contain corrosion inhibitors that may react with some types of soil.

6. How to fix bulging: step-by-step instructions

If the defect has already appeared, follow the algorithm:

1. Scale assessment:
   • 🔍 If the bubbles are single and small (up to 2–3 mm), they can be eliminated locally.
   • 🚨 If the swelling covers more than 30% area, will have to be completely repainted.

Removing the defective layer:

Use soft abrasive (for example, P1000–P1200) or paint remover (for example, Abro PR-600). Do not use metal brushes - they damage the surface.

Retraining:

Degrease the surface, apply a thin layer epoxy primer and let dry 1–2 hours.

Repainting:

Use paint from the same manufacturer, as the soil. Apply 2-3 thin coats at intervals 15–20 minutes.

If bubbles appear after 1–2 days after painting, this is a sign under-film corrosion. In this case you need:

1. Remove paint down to metal.
2. Treat the surface rust converter (for example, Tsinkar).
3. Apply acid soil (for example, Reoflex) for passivation of metal.
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To remove small bubbles you can use needle and syringe: puncture the bubble, squeeze out the air, then fill the cavity with a drop UV gel (for example, 3D Lac) and polish.

7. Prevention: how to avoid bloating in the future

To prevent the problem from recurring, follow these rules:

• 📋 Keep a painting journal: record the materials used (brand of paint, primer, solvent) and conditions (temperature, humidity).
• 🧪 Test compatibility: Before full painting, apply paint to a small area and observe for 24 hours.
• 🔄 Follow the "1/3 rule": first layer - 30% coverage, second - 60%, third - 100%.
• 🛠️ Use professional tools: instead of household spray cans, take spray gun with compressor (for example, Wagner W 550) for more uniform application.

If painting is carried out outdoors, keep an eye on weather forecast. Even light rain 2-3 hours before work can increase the moisture content of the metal, which will lead to swelling. The optimal time for painting is 10:00–16:00 on a sunny day without wind.

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The most common mistake beginners make is saving on materials. Cheap paint in a can can cost 2–3 times less, but the risk of defects increases 5 times. In terms of repeated work, “savings” turn into losses.

FAQ: Answers to frequently asked questions

Can I spray paint over old paint without primer?

It is possible, but only if the coating is old holds firmly and has no chips. Before painting, be sure to matt the surface with sandpaper. P500–P600 and degrease. The risk of swelling remains - about 20%. To guarantee it is better to apply adhesive primer (for example, Novol 420).

How long after priming can I paint?

The time depends on the type of soil:

• Acrylic primer: 15–30 minutes (until matte).
• Epoxy primer: 1–2 hours (full curing - 24 hours).
• Acidic soil: 10–15 minutes, but a layer of acrylic primer is required on top of it.

It is not recommended to speed up drying with a hairdryer - this can lead to “boiling” of the soil.

Why does paint swell a week after painting?

This is a sign under-film corrosion or late chemical reaction. Reasons:

• Poor anti-corrosion treatment of metal before painting.
• Using cheap primer that did not insulate the surface.
• Moisture getting under the paint (for example, when washing a car until it dries completely).

Solution: remove paint down to metal, process rust converter and repaint in compliance with technology.

Is it possible to spray paint in a cold garage in winter?

It is possible, but with reservations:

• Use winter paint (for example, Motip Low Temp), which dries at +5°C.
• Warm up the part infrared lamp up to +18–20°C.
• Increase drying time between coats by 1.5–2 times.

Without heating, the paint will dry at least 48 hours, and the risk of defects will remain high.

Which spray can is best to choose for painting the hood?
For large surfaces (hood, roof) we recommend:
• SprayMax 2K — a two-component system with a hardener, gives a durable coating.
• Mobihel Helios - good hiding power, suitable for metallics.
• Dupli-Color - a budget option with a wide palette of colors.

Important: for the hood you will need 3–4 cans (400 ml each) for one full layer.