Paint technology has moved on a long way in the last 50 years. Here's what it means if you're planning on respraying your classic car
Old cars were always painted in cellulose, right? So that’s what you want to use on your car for its new finish. None of this two-pack stuff you’ve heard about. Makes the car look like it’s been dipped in plastic.
The truth is rather different. Few new cars have been factory-finished in cellulose since the 1950s, and even the repair trade had largely stopped using it by the late ’70s. However, on its invention by Du Pont in 1923, it changed painting dramatically. No longer was production held up while hand-brushed coats individually dried; the nitrocellulose could be sprayed quickly by DeVilbiss’s new invention, the spray gun; it dried quickly, and it gave a good finish.
But there were snags. Certain solvents, including petrol, could attack it. And the repair trade found it very unforgiving of imperfections in the surface beneath.
From the 1930s, car makers moved to enamels that, unlike a lacquer (which hardens purely by evaporation), additionally hardened by combining with airborne oxygen. Baking the freshly painted bodywork in a paint oven as the reaction occurred gave a harder finish. Initially these enamels were alkyd-based, and these were the norm until thermoplastic acrylic lacquers arrived in the 1960s.
Vauxhall was first in the UK with this. It worked by heating up the just-sprayed car bodies enough to melt the paint, so it could re-solidify in a super-glossy form. This worked particularly well with metallic paints, earlier examples of which had tended to dull as the pigments oxidised and the aluminium particles became exposed.
So now we had the ingredients for vibrant new finishes, made more durable when acrylic lacquers were ousted by acrylic enamels. And that, in new cars, is where we are today, the liquid in the paint now water rather than solvent.
For the bodyshop and restoration trade, things are different because you can’t bake a built-up car as hotly as you can a bare shell. So it mostly uses ‘two-pack’ paint, typically based on acrylics and urethanes and mixed before use with a hardener containing very nasty isocyanates, which paralyse the respiratory system. That’s why paint-sprayers wear breathing apparatus.
There are ‘direct gloss’ two-packs that don’t need a clear coat, and give a fairly original-looking finish on your originally alkyd or acrylic-sprayed car. And they don’t have to look thick or plasticky. Not if your paintshop knows its stuff.
Words: John Simister