In spray painting, "mottling" and "clear coat failure" are common points of frustration for many beginners and even experienced spray painters—following the process, the finished product still looks like it has a "dirty filter," either with uneven reflection or as rough as sandpaper. Today, we'll break down these two problems in the most down-to-earth terms, from the causes of mottling to clear coat techniques, all practical details that can save you from half the trouble.
1. Mottling
Mottling (often called "mottling" or "blemishes" in the industry) is like a "visual patch" on the coating, essentially a conflict between the distribution of glitter pigments, coating thickness, or gloss. Let's break down two common pitfalls one by one:
1.1 Uneven Spraying of Silver Powder
When spraying silver powder paint, many people overlook a detail: even a slight difference in the spraying conditions of the base coat can drastically alter the final result. For example, if the spray gun distance varies (sometimes 15cm, sometimes 30cm) and the air pressure fluctuates, the aluminum powder particles will "stray" – some will lie flat on the coating surface, while others will be angled and embedded in the paint film. When light shines on them, the reflections from different angles create spots of varying shades, like irregular stripes "drawn" on the workpiece.
Many beginners panic when faced with this problem, and simply re-spraying only makes things worse. Actually, there's a clever way to salvage the situation: when the silver powder coating has reached 50%-70% of its original gloss (it feels slightly moist but not sticky to the touch, and a light scratch with a fingernail will leave a shallow mark without damaging it), apply a layer of metallic paint using a "medium-dry spray" technique. The specific steps are: adjust the spray gun air pressure to 1.2-1.5MPa, maintain a gun distance of about 25cm, control the moving speed at 40cm per second, and ensure that the dry film thickness difference between each area of the coating does not exceed 20μm (approximately the thickness of two A4 sheets of paper). This step is equivalent to "re-arranging" the aluminum powder, which can make the reflection more uniform and consistent, saving half the paint compared to directly repainting.
1.2 Silver Powder Paint Reflow
"Applying clear coat immediately after base coat" is a common mistake for beginners—the silver powder particles are encased in clear coat before they've "settled" in, mixing with the clear coat layer, causing blemishes to be "welded" directly onto the coating. I once helped a friend repair a DIY painted car; he was too lazy to wait for it to dry, resulting in silver powder reflow that caused "rainbow patterns" all over the door, ultimately requiring the entire car to be sanded down and repainted.
Preventing this is simple: "Wait." After spraying silver powder paint, let it sit for at least 15-20 minutes (an additional 5 minutes if the humidity exceeds 70%). Gently press the coating with your finger; if you can pull out fine threads but it's not sticky, the base coat is completely dry. If you're in a hurry, you can use an infrared heat lamp to speed things up (4 minutes for medium-wave infrared, 3 minutes for short-wave), but never use a hairdryer to blow directly on it—the hot air will make the aluminum powder more scattered, exacerbating the blemishes.
2. How to spray clear coat?
Clear coat is like a "beauty filter" for the coating, but if not sprayed properly, it can become a "disaster"—either running like "tear streaks" or as rough as sandpaper. Remember these two "hands-on" steps, and even beginners can achieve a mirror-like effect:
2.1 Clear Coat Mist Spray
The first coat of clear coat must be "mist sprayed." The key is controlling the viscosity and spraying technique. First, thin the clear coat to a viscosity of 20-25s (using a Coat-4 cup, it means the paint should flow completely from the cup's rim in about 20 seconds). Too thick, and it will clog the spray gun; too thin, and it will drip.
When spraying, adjust the air pressure of the spray gun to 1.0-1.2 MPa, maintain a gun distance of 20-30 cm, and increase the gun speed by 30% compared to normal to form a "semi-transparent veil." Here's a "fluorescent light inspection method": after spraying, shine a fluorescent light on the surface. If you see a slight sheen but no reflection, the thickness is just right. If the reflection is too strong, it's too thick; wait for it to dry completely and then lightly sand it with 2000-grit sandpaper. If there's no sheen, it's too thin; just apply another layer.
After the first mist spray, don't rush to apply the second coat. Lightly touch the coating with your finger; if you can pull out fine threads but it's not sticky, then continue (usually wait 5-8 minutes). I've seen people touch up immediately after spraying, resulting in the clear coat and base coat mixing together, creating "fisheye" effects, and requiring 3 hours of rework.
2.2 Clear Coat Finishing Spray
When spraying large areas of workpieces (such as an entire steel plate), spraying the whole surface directly = "a guaranteed disaster." Try the "three-segment method," which I've personally tested and found can reduce the defect rate to below 5%:
Segmented Spraying: Divide the steel plate into three parts: top, middle, and bottom. Spray the top 1/3 first, slowing down the spray gun speed (20cm/s) at the junction with the unsprayed area to allow the coating to transition naturally.
Immediate Inspection: After spraying the top section, check the texture with a fluorescent light. If it matches the sample, then spray the middle section. If there are rough spots, lightly sand with 2000-grit sandpaper and respray.
Overlapping Spraying: When spraying the middle section, overlap the spray gun with half of the top section's area. After spraying, check the texture again. If there are no problems, then spray the next section.
3. Environment + Tools
Many people attribute spraying problems to "poor technique," but the environment and tools account for at least 40% of the impact:
Temperature and Humidity: The most suitable temperature is 20-25℃, and the humidity is 50%-70%. Below 10℃, varnish dries slowly and is prone to dripping; above 85% humidity, the coating will turn white (moisture condenses in the paint film). I sprayed it once during the rainy season in southern China, with 90% humidity, and the entire coating turned white, requiring complete scraping and repainting.
Spray gun maintenance: After each spraying session, the spray gun must be thoroughly cleaned, and the nozzles cleaned with thinner—I've seen people use a wire to unclog clogged spray guns, resulting in deformed nozzles and "crooked fan-shaped" paint sprays, creating spots.
Mask application: Use masking tape to cover areas that don't need to be sprayed, and smooth the edges with a scraper—if the tape is loose, paint will seep in, requiring an hour to clean.
4. Summary
Spraying is not about "aggressive operation," but a delicate process of "slowly adapting" to the paint: Silver powder should be allowed to "settle" before applying varnish, and clear coat should be applied thinly and allowed to dry slowly for even application; large areas require "segmented spraying + immediate inspection." Remember these key points: Uneven silver powder: Apply touch-up paint during medium-dry spraying, ensuring aluminum powder is neatly arranged; Silver powder reflow: Let it sit for the required time, allowing the base coat to dry completely before applying a top coat; Gloss varnish mist spraying: Adjust the viscosity for 20 seconds, check the gloss under a fluorescent light; Large area spraying: Apply in three sections + check immediately, ensuring consistent texture before proceeding.
Practice a few times to get a feel for the process, and you too can achieve a "mirror-like" coating—after all, good spraying results are never "fast," but rather "wait and check."
