Metal Spraying

Work outdoors or under forced extraction. Wear a half-face respirator with P2/P3 particulate filters and a face shield.

Molten zinc gives off zinc oxide fume. Breathing it causes metal fume fever — fever, chills and aching some hours after exposure. It passes in a day or two and it is entirely avoidable. A dust mask is not a respirator and will not stop fume.

Grit-blast the steel plate with aluminium oxide abrasive to a uniformly matt, angular profile. Spray within 4 hours, before the fresh surface re-oxidises.

A sprayed coating is held on mechanically — the droplets key into the surface texture. There is no alloying and no diffusion bond, so on a smooth surface it simply peels off in sheets. Surface preparation is the process.

Melt 200 g zinc dust in the iron crucible. Zinc melts at 419 °C; hold it at roughly 450–480 °C — hot enough to stay fluid, not so hot that it burns off.

Skim the grey dross off the surface before spraying. Dross in the stream blocks the nozzle.

Feed the melt into a stream of compressed gas at 4–6 bar, directed at the plate from 150–200 mm.

The gas does two jobs at once: it tears the liquid into droplets, and it carries them to the work. Schoop's specification is explicit that the spray is "mixed with steam or hot gas under pressure" — steam was his own first carrier.

Hold the stream perpendicular to the surface. Spraying at a shallow angle lands the droplets sideways, and they build a porous, weakly-keyed layer.

Sweep steadily across the plate, overlapping each pass by half its width. Build up in several thin passes, not one slow heavy one.

Target 100–150 µm total for atmospheric corrosion protection.

Feel the back of the plate between passes: it should stay merely warm. That is the point of the process — the metal is molten in flight, but each droplet is small enough to dump its heat on landing, so the workpiece is never heat-treated by the coating.

Look at the surface: matt, grey and granular — visibly rougher than galvanizing or sherardizing.

Cross-section a test piece if you can. A sprayed coating is a stack of flattened droplets with oxide skins between them and some porosity, not the solid continuous metal a dipped or diffused layer gives.

That porosity is why sprayed zinc is usually sealed with a thin paint or wax afterwards, and why the same structure is an advantage elsewhere — porous sprayed coatings hold lubricant, which is why the technique is now used to rebuild worn bearing journals.

The date almost everyone gets wrong. Nearly every account says Schoop "patented metal spraying in 1909". The British specification, GB 191005712, was filed 7 March 1910 and published 26 January 1911. The 1909 date belongs to his earlier German and Swiss filings, which are separate documents with separate numbers. Both statements can be true at once, but citing "GB, 1909" is wrong.

The origin story is probably tidier than the truth. Schoop is said to have got the idea watching his son fire a toy cannon at a wall and noticing the lead shot flattened and stuck. It is repeated everywhere and it traces back to Schoop's own later telling. Treat it as an anecdote, not a documented event.

What came after. The 1910 process sprays from a melt. Schoop's next development fed wire into a flame and atomised it as it melted — that is the pistol in every photograph, and it is what made the process portable. Arc spray, plasma spray and HVOF all descend from the same idea: melt it, throw it, let it splat.

Why it survived. Not because it protects better than galvanizing — it does not, on a small part. It survived because it has no size limit and needs no heat in the workpiece. Bridges, lock gates, ship hulls and steel-framed buildings get sprayed zinc or aluminium in place. And rebuilding worn shafts by spraying metal back on is now a larger industry than the corrosion work Schoop was aiming at.

About the photograph on this blueprint. It shows a wire-fed spray pistol — the later development described above, not the 1910 molten-bath apparatus this blueprint follows. It is the clearest authentic photograph of metal spraying available under an open licence, and the physics it shows is identical: melt, atomise, splat. The feedstock is what differs. Said plainly here so the picture cannot imply what the text denies.

Honest limitation. Adhesion is purely mechanical. A sprayed coating on inadequately prepared steel will pass inspection and fail in service, which is why blast standards for spray work are unusually strict.