TWO PACK PAINT
The subject of paint comes up all the time in discussions at club meetings,rallies etc
But what do we really know about it? well, the following should give an inside view to the topic.
Two-pack, Twin Pack, or ‘2K’ as it’s sometimes known, is a type of automotive paint which has been around for years now, replacing the old cellulose, and is widely used on both production cars and in modern repair bodyshops.
Automotive refinishing’ (posh for respray) is a complicated subject, made worse not only by the jargon, but by the constantly changing technological advances. Basically, all that Vehicle paint is, is a clear resin which has a pigment (or colour) suspended in it. At the factory, the resin has coloured powder suspended in it to form lots of basic colours, the reds, blues and greens etc. Then the basic paints are distributed to the retailer, who has a Mixing Scheme, i.e. a machine which can measure exact quantiies of the basic colours, combining them to produce the final colour.
Metallic paints follow the same principle, except the clear resin has tiny aluminium flakes suspended in it. And to this the retailer adds the basic colour mix to create the final tint. (Of course, with metallic finishes you have to spray a clear lacquer as the final topcoat).
So that’s basically what car paint is - the complicated part is the clear base resin, which changes with different types of paint. The original automotive paint was of course cellulose (or Nitro Cellulose Lacquer). Cellulose is the oldest type of finish; Henry Ford used it on the original Model T, and its still much used by non-professional sprayers.
It works or dries by solvent evaporation, as the Coloured paint is thinned down by large quantities of cellulose thinner which evaporates when the paint is sprayed onto the metal surface. Two-Pack is a completely different process. The paint consists of different resin, made of acrylic and melamine (the proper title is Two Pack Acrylic Enamel). When you mix your paint, you add a second resin, or hardener, called Poly-isocyanate Resin. At this stage a chemical reaction takes place between the two resins which makes them harden. Heat can increase the speed of the reaction and a modern paint sprayshop may use an oven to decrease the drying time to as little as 40 minutes.
So, the Advantages and Disadvantages? The biggest problem with cellulose is in the thinners - with up to 50% of its’ sprayed volume evaporating, you actually need to apply a lot of coats to get a decent final build-up of paint. But because Two-Pack only contains a tiny amount of thinners, what you spray is what you get when the paint has finally cured, leading to far less wastage.
When Two-Pack has cured it is also much harder than cellulose and tends to resist petrol, acid rain, sunlight etc. much better than cellulose, which oxidises and goes dull over a long period, or even develops a milky haze.
On the other hand, Two-Pack is strictly a ‘professional-only’ product. The hardener contains isocyanate, which is extremely toxic if breathed in, and expensive spray-booths and air-fed breathing apparatus must be used. Cellulose of course doesn’t require this; a simple filter mask will do.
As far as classic car work is concerned, Two-Pack is a godsend. Because the primer also contains little thinners, several coats can be used-to build up a heavy layer, which can eliminate the need for primer-fillers and spray putty when your body panels are not 100% flat.
When painting over old cellulose, which you almost always do with our
cars (unless you’re stripping to bare metal) solvent reactions are often a big problem. The solvent or thinners in the new paint can react with the old paint, causing blistering and sink marks. Because Two-Pack has less solvent, it tends to be much more inert and won’t react so badly.
So that’s the current state of play, even though its only a brief look into the subject. There are more paint systems being developed ail the time, the latest being a water-based paint, and several manufacturers have introduced non-isocyanate hardened Two-Packs, which would eventually be ideal for the home restorer if you’re thinking of respraying your car yourself in the, next year or so, a visit to your local vehicle paint distributor for some advice and a few information brochures might be a very worthwhile investment.With acknowledgements to Mike Wilkinson of M. WRestorations.