Synthetic and enamel paint explained

I hope one or more of you chaps can explain the basic theory to home bursh and spray painting metals. This topic could well be unclear to other noobs, so have tried to lay my questions out clearly.

Is my below research correct? Please confirm each in turn

1. "Cellulose" paints use cellulose thinners to dissolve the binder. Nitro-cellulose based. Fast drying by evaporation of the solvent (contrast to synthetic)
    
2. Cellulose was the traditional 20th century paint for machine tools aka "coach painting" applied by brush. Common to apply many thin coats
    
3. Cellulose has been superseded by "synthetic" (exclusing 2 pack) industrial painting
    
4. "Synthetic" and "enamel" mean same thing. Chemically based on alkyds.
    
5. Synthetics dry by a chemical hardening, hence take much longer to fully harden than cellulose (via evaporation). Results in synthetic being more hard wearing.
    
6. Synthetics are best thinned for spraying with their own brand thinners, cos their chemistry can vary wildly between paint families.
    
7. Brushes with synthetics can be cleaned with cellulose thinner (seems to be wrong...?)
    
8. Synthetic under & topcoats should be from same product family to ensure proper bonding.
    
9. Aluminium needs a suitbale etch primer, to key properly. It is then overcoatable with synthetics or cellulose, but don't mix them on subsequent coats.

Thanks for reading this far!

Edited By choochoo_baloo on 27/05/2020 01:53:07

Edited By choochoo_baloo on 27/05/2020 01:53:55

my very limited knowledge is as follows.

as i understand it, synthetic enamel paints were commonly used for brush painting , they would be used for coachpainting commercial vehicles, or for general metal painting, if you got a tin of bog standard paint to paint say a metal gate, that would be synthetic enamel because such paint is suitable for brush painting. cellulose based paints were more commonly used for automotive re finishing because they were suited to spray application, though on cheap jobs on older cars, synthetic paint might be used to cheapen the job, the paint itself was cheaper, only required one coat and because it went on thicker was more forgiving and required less preparation.

you can paint synthetic paint over cellulose, but not cellulose over synthetic. aluminium needs an etch primer to key the paint onto the metal, once you've applied the etch primer you can put a coat of normal primer over it. synthetic enamel will dry over night but then takes a couple of weeks or more to go fully hard, this means if you paint a car with it, you can't polish the car for weeks afterwards, and if you get a run in the paint you can't do anything to rectify this until the paint is hard enough to flat it out.

you can clean your brushes with cellulose thinner or gun wash if you've used synthetic paint,it will work as a solvent to dissolve the paint and any reaction doesn't matter, dry the brush before you use it again though so as not to contaminate the next job. the reason for doing this is because the basic cellulose thinners will be cheaper than the synthetic thinner, you wouldn't buy a couple of litres of brand specific synthetic thinner then use 90% of it for brush cleaning. Anyone who does a lot of automotive painting will have a drum of low grade cellulose thinners or gun wash to clean equipment after painting and would use it across the boar for cellulose and synthetic paint, if you're just doing a bit of brush painting, some white spirit would be a more cost effective option though.

these days very little commercial painting would be done with cellulose, it's was superseded in the 90's by two pack paints that gave better, harder finishes, but were pretty toxic and not for non professional use, these in turn have been superseded by water based paints.

A couple of points - the traditional coach painting is as you describe, but the usual brushing paint for machine tools was an oil based enamel.

Natural oil based enamels have broadly been superseded by manmade alternatives - hence the term synthetic.

Apart from the water based paints mentioned and the poisonous 2 pack isocyanate paints, both designed for spray application and low temperature oven bake to finish there are cellulose and synthetics.

Rather than trying to brush the paint I would suggest aerosols are the way forwards. Have a look at this site - a midlands based chain who happen to be close to me, but there will be similar near to wherever you are. **LINK**

Apart from the vast range of car colours have a look at the BS and RAL colours to find a match. The company will then mix it up and fill an aerosol for you. I would suggest synthetic, but it is forty years since I worked in the paint supply business (as a technician supporting auto refinishers, mixing and matching paint and troubleshooting) and the chemistry and technology has almost totally changed since then.

Ask at your supplier - if your experience is like mine they will be pleased to help. If you take in another part of the machine they will also have paint 'chips' to help you choose the right colour.

Edited By Nick Clarke 3 on 27/05/2020 09:25:34

2. Painting cars used to take weeks back in 1900. The reason Henry Ford said 'you can have any color as long as it's black' was it dried the fastest. Nitro cellulose paint was developed to get around this issue.

6. Very true, you can get fast synthetic thinners and not all synthetic thinners will thin all synthetic paint. If you have ever tried thinning new Hammerite paint with old hammerite thinner you will understand the effect. You need to rely on the paint supplier for matching products.

7. Cellulose will thin anything including water, which is why gunwash is just for cleaning sprayguns. It seems to be OK, but leaves a lot of water trapped in the paint, at the first frost it freezes and gives you lots of tiny blisters in the paint.

They say the gloss on older synthetic is only on the surface so you can't flat paint runs and polish the gloss back. Halfords used to sell a product in the 70's called Repaint which was a brush paint for cars. You needed huge skill to use it and the best way to devalue a car that I can think of.

Choo, I agree with your thoughts to want to try and understand what goes on with paints. We need someone who has written a book on the history of paint to comment, but I doubt there is one.

I agree with some of what you have said, but not all. Here is my summary of understanding. I have messed with paints for forty years but I have no specific competence in the subject.

1. Traditional paints were called 'enamels' and were pigments ground up with oils and resins of animal and plant base. Designed for brush application. The solvent was turpentine and more recently white spirit.

2. Natural materials have been replaced by synthetics (chemical industry products) and the word alkyd appears. Synthetic enamel can be thinned with white spirit but synthetic thinners is available. Toluene is a constituent. Synthetic enamels dry more quickly (which can make brushing more difficult) and are relatively easy to apply by spray because of this.

3. Cellulose paints were (I think) developed purely for spray application (1930's?) and were quick drying for industrial speed. You cannot brush apply because of this. The solvents are based around cellulose acetate. Cellulose thinners is a powerful solvent and will dissolve and melt most other paints. Hence, never put a cellulose paint on top of other paints or primers.

4. The automotive industry moved away from cellulose base to polyester many years ago. You cannot buy cellulose paints now (generally) and for home spraying it is polyester and a slightly more friendly 2K thinner. Gloss polyester is mixed with an activator (urethane or isocyanate reaction?) to make it harden. The solvents evaporate more quickly than cellulose. It sprays easily, drys fast and is much more durable than cellulose (modern petrol resistant) The majority of car respray systems involve a 2K clear lacquer topcoat. Safety breathing equipment HAS TO BE WORN. The car world has moved to water based paints and this will also arrive for home use, which will be a good thing for spraying, but not so for brushing perhaps.

The above relates to top coats. There is another subject on etch primers and on undercoats.

Edited By norm norton on 27/05/2020 09:58:53

A couple of points - Cellulose paint is still widely available in most colours, but commercially is not permitted except for heritage vehicles and repair work on existing cellulose. Amateurs can still use it. - see the link in my first post for one source of availability.

While cellulose paints were in the main sprayed brushing versions were available (the best known was brushing Belco) and brushing additives were, and I believe possibly still are, available to slow down the drying of cellulose to a point where they can be brushed. This was a traditional coachpainting method using many, many coats rubbed down with very fine abrasive in between each.

Despite all of which, I suggest that for the purpose of repainting a guard on a machine, synthetic is the way to go.

I would say that traditional nitrocellulose and oil-based enamels are traditional paints.

Synthetic paints are polymer based formulations like acrylic and polyurethane that effectively produce a 'plastic coating'.

The best thing about nitro paint is that it ages beautifully, its colours often fade, it loses its sheen and gets a lustre instead and, on wood, it can 'check' into lovely patterns as a result of temperature cycling.

Synthetics are more hard wearing, thicker and tend to have a deeper shine.

Enamels tend to fall between the two.

I am biased by thinking about musical instruments... a forty year old nitro finished one will have aged beautifully, a forty year old synthetic finish may look almost as good as the day it was painted. Which is better is a matter of taste!

For painting my home made tools and accessories I always use enamels for their oil resistance and general toughness with easy application. For models, it varies.

Neil

I'm only an amateur spray painter and have painted a car with celulose type paint which required a lot of work to get a good finish. Lots of rubbing back between coats and buffing the final coat to get a shine. But it does look good.

I've just painted my traction engine with 2 pack paint which is much quicker. The panels were sand blasted and an etch primer applied, followed by a normal 2 pack primer and rubbed back with wet and dry paper. Then 2 colour coats. No clear coat and no final buffing required for a shiny finish. Probably not as good as I achieved with the celulose paint by applying a clear top coat, rubbing it smooth with fine wet and dry and buffing to get a shine but trying to cut and buff around all the rivet heads on my engine would be almost imposible.

But as mentioned, you do need to wear the correct PPE because it is cancerous if you breath the fumes. I also did the painting outside.

Paul.

Under the Control of Substances Hazardous to Health Regulations 2002 (COSHH), exposure to isocyanates must be controlled in order to protect the health of workers- this is due to the fact that isocyanates are respiratory sensitisers and are known to cause occupational asthma, bronchitis and pneumonitis. Early symptoms of exposure may include shortness of breath, flu-like symptoms and a ‘tight chest’ feeling. Although symptoms may improve after the irritant is removed, acute asthma attacks may occur after renewed exposure to isocyanates; even if the exposure is very small or very brief.

Other health effects of exposure to isocyanates include irritation to the nose, eyes and throat, gastrointestinal irritation and inflammation of the skin (dermatitis). Dermal sensitivity as a result of overexposure to isocyanates that may result in a rash, itching, hives and swelling.

Lifted from the web.

2K is wonderful stuff

When cars were painted with cellulose it was normal to polish with a fine compound (eg Farecla) to provide the shine. When synthetics came in in it needed a whole new way of working because this type of paint would not polish in the same way - being slower drying under the surface it was still soft when cellulose would be hard.

The good part was that a good gloss was obtainable right off the gun so the polishing was not needed, saving labour and time.

Clear topcoats were not normally used in the 1970s with the exception of some expensive German makes that used special paints where the lacquer was applied wet-on-wet to lift the flake into the lacquer slightly emphasising he metallic effect - examples of this were Galsurit 54-line and 94-line paints.

A few painters tried to use lacquer on cellulose to save on the time consuming polishing, but cellulose lacquers were not very stable and tended to yellow, and in extreme cases crack with exposure. It was not until the arrival of acrylic lacquers (initially still thinned with cellulose thinners) that they became more common. These were very strange to spray filling the sprayshop with what looked like hundreds of spiders webs that settled into the finish and became an invisible part of it.

Today it is all very different with not only metallic colours but solid colours as well needing a lacquer to seal the paint and provide the final shine.

Edited By Nick Clarke 3 on 27/05/2020 12:00:51

Non isocyanate 2K is available and is also good, and that as far as my nervous disposition will allow me to go!

Great input chaps. Always appreciated.

Follow on questions if I may:

1. In simple language, what is "1K", "2K", "2 pack"
   I thought that 2 pack meant a paint + isocyanate mixed into the gun pot and then sprayed.
   (Yes I heeded the dangers of isocyanate a while back - I won't be using it any time soon!)
   
2. After the preceding discussion, for (a) hard wearing (oil resistant, won't flake if eg allen key dropped) (b) brush paints (c) with a wide choice of finishes, can we agree that synthetic (like the Selemix I've already used) should be my first choice?

1 k is a paint that is applied & sets

2K & 2pack which are the same require a activator to make it set. So with a 2k you would put in the vessel, add the required amount of thinners, usually 50%. Then put in the activator/ hardener. This will give you a certain amount of time to apply the paint before it begins to harden. When spraying cars & motorcycles I used to wait 15 mins between coats. Once finished it needs leaving overnight to fully cure. My son in law has a Paint booth which is temp controlled. Once set up he can paint a car & within one hour it is fully cured.

Of course as some have already said cellulose paints are now banned. As is icocyanate based. The stupid thing is that, like my son in law has to abide by.

He paints a car or motorcycle race fairing etc. He has to by law use water based paint, which he hates.

He then applies the clear coat,(Clear laquer).Clear laquer only comes in 2 pack isocyanate. Which is deadly.

All these paints are not oil based like the paint you would probably use to paint a model with, which can be applied by brush & has a setting time much longer than automotive paints. Clear as mud.

Just to point out coach paint was not cellulose. It was brush applied. The cellulose goes of in a matter of seconds. By that I would say , when I painted cars way back with it in warm weather. Probably less than 2 mins to touch dry. 

Steve.

Edited By Steviegtr on 29/05/2020 02:34:20

The OP asked for a history of paints so hopefully this post will clarify a few things. Historically until about 1900 all paints belonged to a category called oleo-resinous varnishes. In other words they were linseed oil or similar, in which was dissolved a natural resin e.g. rosin, kauri gum etc, plus pigment and solvent. These paints dry by oxidation which means that they do not become hard enough to sand for a few weeks.These paints have two drawbacks 1. that they yellow on ageing and 2. they crack over time to produce an aligator-skin like effect that will be well-known to the restorers of vintage vehicles. Because of their tendency to darken the so-called heritage colours on victorian buildings tended to dark greens and reds, browns and also cream (where yellowing was not so obvious). They are well and truely obsolete nowadays.

Cellulose paints appeared around 1900 and are fast-drying, have high gloss and are reasonably durable. The explosive properties of the guncotton used to make them and the extreme fire hazard of cellulose paints meant that the paint manufacturers heaved a sigh of relief when alternatives became available and promptly dropped cellulose paints from their lists.

In 1930 paints were placed on the market where the natural resins in oleoresinous paints were replaced by a synthetic resin (alkyd paints). These had excellent colour stability, durability and were cheap. Therefore they replaced the oloeresinous paints almost overnight. These paints were universally used from 1930 up until about 1960 and still have a big use today. It is worth noting that they are only partially synthetic because they still use Linseed or similar oils in their chemistry

The alkyd paints because of their hardness and high gloss were given the trade name enamels in a reference to vitreous enamels. The word enamel in a paint context is misleading and still causes confusion when used for some modern paints. It is a term best forgotten!

Around 1960 many types of paint became popular in place of the universally used alkyd.

Acrylic paints were popular for a time and use what is essentially perspex dissolved in solvent. They are the modern equivalent of nitrocellulose without the hazards and give high gloss and very good durability. They require powerful solvents and can be quite expensive.

The 2-pack isocyanates give a performance previously unobtainable in terms of gloss retention, colour retention and durability. Their toxicity is a problem. Safety precautions should be followed rigidly with these paints but the risk is possibly quite low for most people. As with other toxins, individual susceptibility is a major factor and you might be more-or-less immune. On the other hand you might be ultra-susceptible and you won't know this until it is too late (this is why some people smoke cigarettes heavily to a ripe old age while other smokers succumb to cancer in their 30's).

Through the 1980's to about 2000 polyester paints were widely used for cars etc. These are alkyd paints where the chemistry of the base resin has been modified to replace linseed oil. Thus they are fully synthetic but need precise application conditions to succeed and as far as I know they are not sold for retail purposes.

Of late there have been major efforts made to move to water-based systems for ecological reasons. Except for some industrial applications which again are carried out under closely controlled conditions (and the very popular latex house paints which are a different animal),these are not much cop in my experience for hobby or retail use.

If I was finishing a model I would use an alkyd paint or a 2-pack isocyanate depending on how the model was to be used.

Anyone care to comment?

Chas

Thank you Charles for that fuller description of paints, I see that there are several forms of 2 pack paint, some being "safer " and not containing isocyanates. Could anyone care to suggest the better paint for model locomotives where oil and "hot" surfaces would be found ie boiler painting where you may expect perhaps 120deg C ??

Also comment on the difference between Acrylic and Polyeurathane paints.

Thanks

As I have already said, it is a long time since I was involved with auto refinishing, however I was very technically familiar with the products used then, if not those in use today. Cellulose paints with a high volatile organic content (VOC) level are indeed prohibited under an EU directive for vehicle refinishing. (but which may or may not have been enacted yet under UK law) Technically the sole exception is that they may be used for vehicles of significant historical value, adjudged to be so by a competent authority and then an individual licence for their use may be granted.

EU Document on VOCs

However this is the actuality as published by a paint supplier, there are many others as an internet search will reveal.

Guide to using cellulose paints

The HSE accepts the use of cellulose paints in their advice document 'HSE Health and safety in motor vehicle repair and associated industries' - "117 The health hazards of solvent-based conventional spraying products (commonly known as ‘cellulose paints and include irritation of the eyes, nose and throat, and mild, reversible effects on the body" Incidentally this is the only reference made to cellulose in HSE literature and post dates the EU regulation.

Regarding coach paints, I suspect you may be confusing coach painting which is a long established skill using different paints, including enamels and cellulose, with coach enamel which is an oil based (now synthetic) product for carrying this out.

Brushing cellulose paint was available until the mid seventies, as were inhibitors to slow down the drying and make cellulose brush able I sold them! Here is an advert from 1958 for the best known brand.

Edited By Nick Clarke 3 on 29/05/2020 10:00:43

Engine laquer

And on smokeboxes and chimneys wood burning stove black paint has been successful on our club locos.

Pedant alert!

Enamel originally was a baked on surface, typically powdered glass made hot enough to melt and stick to a metal. Very tough, originally used for jewellery, then kitchenware and other hard wearing metal items. It's not really a paint at all.

Enamel Paint is any sort of paint whatever that looks a bit like genuine vitreous enamel. A cynic would say Enamel Paint is a cheap inferior substitute for the real thing. An apologist would point out good looking results are achieved without special equipment.

However, as Enamel Paints aren't a particular paint, or even a family of related paints, you can't generalise about thinners, application, wearing properties, suitability, colour fastness, or anything else. Always read the instructions! Brand X and Brand Y may be completely different.

Is paint technology the most complex engineering subject ever discussed on the forum? It's got to be a candidate!

Dave

Edited By SillyOldDuffer on 29/05/2020 13:24:02

No - I think the guys who use the domestic dishwasher for parts cleaning and the domestic oven for heating bike crankcases, even if the bike in question belongs to SWMBO, have far, far, more explaining to do on how such unbelievable things came to pass!

My advice is always to go to a specialist supplier, either online or even better face to face, and ask for suitable products for the task in hand.