Welcome to Surface Treatments and Finishes!
Ever wondered why a wooden fence doesn't rot immediately in the rain, or why your smartphone screen looks so shiny and smooth? That is all thanks to surface treatments and finishes. In this chapter, we will explore the final "makeover" we give to products to make them look great and last longer. This is a key part of the Specialist Technical Principles section of your AQA GCSE. Don't worry if it seems like a lot of names to remember—we will break it down material by material!
Why do we use finishes?
Before we look at the "how," we need to understand the "why." There are two main reasons to apply a finish:
1. Aesthetics: This is all about the "look and feel." We use finishes to add color, texture, or shine to make a product more appealing to customers.
2. Function: This is about protection. Finishes can stop a material from rusting (corrosion), rotting, or being damaged by the sun (UV degradation).
Quick Review: Corrosion and Oxidation
Materials, especially metals, can react with oxygen and water in the air. This is called oxidation. In ferrous metals (metals containing iron), this causes rust. A good finish acts like a shield, blocking the air and water from reaching the material underneath.
Key Takeaway: Finishes make products look better (Aesthetics) and last longer (Function).
1. Papers and Boards
Think about a high-end chocolate box or a glossy magazine. They aren't just plain paper!
Printing: Adding text and images. Common commercial methods include offset lithography.
Embossing: This creates a raised pattern on the surface. If you run your finger over a fancy greeting card and feel the bumps, that is embossing! It adds a "premium" feel.
UV Varnishing: A liquid coating is applied and then dried instantly using Ultra Violet (UV) light. It creates an extremely high-gloss, shiny finish that protects the paper from moisture.
Did you know? UV varnishing is often used on book covers to make the colors "pop" and prevent fingerprint marks.
Key Takeaway: For paper, finishes are mostly about visual appeal and texture.
2. Timber Based Materials (Wood)
Wood is a natural material, which means it can rot or warp if it gets wet. We have to seal it!
Painting: Adds color and provides a solid barrier against the weather. Great for outdoor play equipment.
Varnishing: Like paint, but it's transparent. This allows the beautiful natural grain of the wood to show through while still protecting it.
Tanalising: This is a heavy-duty preservative process. Wood is placed in a vacuum tank and chemicals are forced deep into the fibers under pressure. This is why some fence posts have a slightly green tint!
Memory Trick: TANalising is like a "sun tan" for wood that stays outside—it protects it from the elements for a long, long time!
Key Takeaway: Timber finishes are essential for durability and preventing rot.
3. Metal Based Materials
Metals are tough, but their biggest enemy is corrosion (rust). We need to "plate" or "coat" them.
Galvanising: The metal (usually steel) is dipped into a bath of molten zinc. This creates a sacrificial layer that stops the steel from rusting. You see this on street lamp posts and buckets.
Dip Coating: The metal is heated and then dipped into fluidised plastic powder. The powder melts onto the metal, creating a thick, soft-touch plastic coating. Think of the handles on a pair of pliers.
Powder Coating: This is an industrial version of painting. Static electricity is used to make dry powder "stick" to the metal, which is then baked in an oven to create a very hard, durable finish.
Common Mistake to Avoid: Don't confuse Galvanising with Painting. Galvanising is a chemical bond with Zinc, making it much harder to scratch off than regular paint!
Key Takeaway: Metal finishes are primarily used to inhibit corrosion.
4. Polymers (Plastics)
Most plastics come out of the mold looking pretty good, but we can still improve them.
Polishing: Using a "buffing" wheel and a fine abrasive cream to remove scratches and make the plastic crystal clear and shiny.
Printing: Just like paper, we can print logos directly onto plastic (like on a soda bottle).
Vinyl Decals: These are essentially "high-quality stickers" made of vinyl. They are used for car graphics or signs because they are weather-resistant and easy to change.
Key Takeaway: Polymers are often finished to add branding or remove manufacturing marks.
5. Textile Based Materials (Fabrics)
Textiles are all about comfort and style, but they need to be practical too.
Dyes: Adding color to the fibers. This can happen at the fiber stage, the yarn stage, or the finished fabric stage.
Printing: Adding patterns or images to the surface of the fabric (e.g., a screen-printed T-shirt).
Stain Protection: A chemical spray (like Scotchgard) is applied to the fabric. It coats the fibers so that liquids "bead up" on the surface instead of soaking in and causing a stain.
Analogy: Stain protection is like a raincoat for your sofa. The liquid just sits on top so you can wipe it away!
Key Takeaway: Textile finishes focus on color and cleanliness.
6. Electronic and Mechanical Systems
Even things inside a machine need finishing!
PCB Lacquering: A clear, protective coating (sometimes called conformal coating) sprayed onto Printed Circuit Boards. It stops moisture from causing short circuits and prevents dust buildup.
Lubrication: Using oil or grease on moving parts (like gears). This reduces friction, which stops the parts from wearing down and prevents them from getting too hot.
Key Takeaway: These finishes are 100% functional—they keep the system working smoothly and safely.
Summary Checklist: How to prepare for a finish
Before you apply any finish, the material must be prepared. If you forget this in your exam answers, you'll lose marks! Don't worry, the steps are usually very similar:
1. Clean the surface: Remove dust, grease, or dirt.
2. Smooth the surface: Use abrasives (like sandpaper for wood or emery cloth for metal) to remove bumps.
3. Degrease: Especially for metals, use a chemical cleaner to make sure the coating will stick.
4. Masking: Cover any areas that you don't want to be finished using tape.
Quick Review Box:
- Paper: Printing, Embossing, UV Varnish.- Wood: Paint, Varnish, Tanalising (Preservative).
- Metal: Galvanising (Zinc), Dip Coating (Plastic), Powder Coating.
- Electronics: Lacquer (Moisture protection), Lubrication (Friction reduction).