MYP Design Surface Finishing Notes

A Finish Has a Job to Do

1. Protection is the most common reason: bare mild steel shows orange spots within days in a damp workshop, and a coat of paint keeps water and oxygen off the iron.
2. Appearance is the second: colour, gloss level and texture all change how a product reads, and a matt black speaker cabinet hides the fingerprints a gloss one puts on display.
3. Grip matters where hands meet the product, which is why a screwdriver handle gets a textured rubber overmould rather than smooth polished plastic.
4. Hygiene decides the finish on anything food or medical: a sealed gloss surface wipes clean, while bare timber holds bacteria down in the open grain.
5. A finish can also unify materials, so a frame made of pine and MDF reads as one object once it is all sprayed the same grey.
6. Pick the finish before you start building, because some finishes need the parts separated and flat, and some can only go on once the product is assembled.

Preparation Sets the Ceiling on the Finish

1. Any scratch that is there before the finish goes on is still there afterwards, and a gloss coat makes it more obvious rather than less.
2. Work down through abrasive grades in order, roughly 80 grit for shaping, then 120, 180 and 240 to get ready for a coating.
   1. Each grade exists to remove the scratches left by the one before it, so jumping from 80 straight to 240 just gives you shiny 80 grit scratches.
3. Sand wood along the grain, because cross grain scratches soak up stain and turn into dark lines you cannot sand out later.
4. On metal the same logic runs file, then draw file, then emery cloth wrapped round a flat block so the abrasive cannot dish the surface.
5. Wipe every trace of dust off with a tack cloth or a barely damp rag and let it dry, since a speck caught under varnish becomes a permanent bump.

Wood Finishes: In the Grain or On Top of It

1. Oil such as danish or tung soaks into the fibres and hardens inside them, leaving a low sheen that still feels like wood and can be repaired by wiping on another coat.
2. Wax buffs up to a soft sheen in minutes but resists almost nothing, so it usually goes over an oiled or sealed surface rather than onto bare timber.
3. Varnish builds a hard film on the surface, and two or three thin coats with a light 240 grit rub between them beats one thick coat every time.
4. Stain is colour with no protection, and it soaks deepest where the wood is most porous, so end grain always comes out darker than the face.
5. Paint covers the grain completely, which is why it is the sensible choice on MDF or on an assembly of mismatched offcuts.
6. Absorbent surfaces need a primer or sealer first, since a cut MDF edge drinks paint and dries rough and furry without one.

Metal Finishes: Keep the Oxygen Off

1. Paint on steel needs a primer under it, because paint straight onto bare steel lifts in sheets as soon as rust starts spreading underneath.
2. Powder coating sprays dry polymer powder onto an electrically charged part so it clings, then bakes it at around 200 degrees Celsius until the powder flows into one tough even film.
   1. School lockers, bicycle frames and garden furniture are powder coated because the film is thicker and harder than spray paint and no solvent is released.
3. Galvanising dips cleaned steel into molten zinc at about 450 degrees Celsius, coating it inside and out including places a brush could never reach.
4. Zinc protects twice over: it is a barrier, and where the coating is scratched the zinc corrodes in place of the steel, which is why a scratched galvanised bin does not rust from the mark.
5. Plastic dip coating heats the part to roughly 180 degrees Celsius and lowers it into fluidised polythene powder, which melts on contact into a thick soft layer for tool handles, dishwasher baskets and playground handrails.

Polishing and Anodising Work the Metal Itself

1. Polishing is just sanding taken further, working down through finer abrasives until the remaining scratches are too small to scatter light.
2. A cloth buffing mop loaded with polishing compound does the last stage, and the compound is a very fine abrasive held in wax.
3. Polished steel is still bare steel and will rust, so it usually gets a clear lacquer over the top to hold the shine.
4. Anodising works only on aluminium: the part is made the anode in an acid bath, and the current thickens the natural oxide layer that aluminium already carries.
5. That oxide is hard and porous while it is fresh, so dye is soaked into it before the surface is sealed, which is where coloured drink bottles and phone bodies come from.
   1. Because the colour sits inside the surface rather than on top of it, an anodised finish cannot chip off the way paint does.

Polymers Get Polished, Not Painted

1. Acrylic is coloured all the way through, so finishing it means taking scratches out of the surface rather than covering the surface up.
2. A sawn edge is cloudy because thousands of tiny facets scatter the light instead of letting it pass straight through.
3. Work up through wet and dry paper used wet, roughly 240, 400, 600, 800 and 1200, with the water floating the debris away so the paper does not clog and score the surface.
4. Finish with plastic polish on a soft cloth, or a buffing mop run slowly, and the edge goes from milky to glass clear.
5. Flame polishing runs a fine gas flame steadily along the edge so the surface melts just enough to flow smooth, and it takes seconds rather than the ten minutes wet and dry needs.
6. Move too slowly and you get bubbles and scorching, and the heat also leaves stress in the edge that can craze into fine cracks if solvent cement touches it later.