In the old days, the art of coachpainting was a much slower affair than it is now. Back then the painting of carriages or motor vehicles took a month or so to complete, from start to finish, using synthetic enamels.
Nowadays, however, the process is expected to take a week or less, the only way this can be accomplished is by cutting down on preparation time and reducing the amount of coats applied.
The number of coats can vary from anything between three and fifteen and the condition of a wooden or metal surface plays an important part in regards to the number of coats required: roughness, porosity etc.
It is true that quality work can still be produced, even when spending less time on preparation and materials.
Nevertheless, the overall finish will still lack the superior shine and colour depth associated with traditional coachpainting standards.
The information provided here attempts to draw together both the theoretical and practical aspects of both coachpainting and vehicle spray painting methods.
The key factors to consider in obtaining a perfectly smooth finish ready for paint, on any surface are:
· Cleanliness.
· Preparation.
· Priming.
· Filling.
· Undercoating.
· Glossing.
· Polishing.
Obviously the smoother the surface the less build up of paint required:
Porous surfaces undoubtedly require more care with preparation so that the layers of paint material built up are sufficient to fill the rough wood or metal surfaces. Wood is a natural fibrous material, on which it is relatively easy to obtain a really good grip for subsequent paint layers to adhere to. This grip can reasonably be described as tenacious if the preparation has been carried out correctly.
Metal however, has always proved to be far more difficult for paint to adhere to, due to the ultra-smooth, often oleresinous surface, This is why modern acid etch primers are used to force the initial grip of primer to stick as tenaciously as with wood.
Synthetic paint consists of the following to make the paint medium:
Pigment solid (colour)
Fluid vehicle (Linseed oil, china oil, alkyd, polyurethane)
Thinners (Turpentine, white spirit)
Driers (metallic salts of, cobalt, manganese, lead monoxide)
Mixed together provides a liquid medium.
The early natural types of pigment were earth pigments, umbers, yellow ochre or sienna.
Ochre which can be burnt to create what is known as burnt umbers which gives different pigment shades.
Mineral pigments such as china clay, chalk or barytes are all white: these particular pigments provide excellent colour fastness and are relatively cheap.
Colour or light fastness means a colour has less tendency to fade in sunlight although this is more likely to effect paint containing cheap pigments not expensive ones.
Lead was once used as the main ingredient in paint, nowadays it is more likely to be found in glazes or ceramics, although lead paint can still be purchased for use on listed buildings.
The pigment solid is suspended in a fluid vehicle that does not dissolve. Oil based paint and varnish dry slowly by oxidation when exposed to oxygen which is when the oil and oxygen combine leading to a hard dry skin.
Modern resin based synthetics: Alkyd for example dry by a process called polymerisation which is a chemical reaction where molecules join together forming one component as the resin and the simultaneous evaporation of solvents (usually turpentine) take place.
It is worth noting that to obtain optimum performance from any paint material (if possible) the paint should be applied at room temperature 68°-72° Fahrenheit 21°-23° Celsius, This temperature not only ensures correct drying and application time it also allows moisture to evaporate completely.
Paint material applied in temperatures too hot will cause the material to become sticky and dry too quickly whereas applying in temperatures too cold or damp stiffens the material causing runs or blooming.
Two finely applied thin coats are much better than one heavily applied thick coat that may give a rough brushy appearance.
Primers:
The term primer has become a much misused name to describe a primer coating, Primer is a primary or foundation coat offering the first layer of protection prior to building up subsequent paint layers.
There are many different kinds of primers which include primer fillers, corrosion inhibiting, adhesion promoting, sealing or isolating primers.
The first coat of primer is the most important layer of any paint structure as it has more than one function to perform. It must bond to the surface with enough strength to allow additional coats to be applied over the top and allow sufficient movement for expansion and contraction of the surface whether it is wood or metal.
The entire painting system or process depends on the priming coat it is therefore in the interest of the coachpainter or spray painter to use a good quality primer designed specifically for the job in hand.
There are several synthetic primers suitable for steel as follows:
Red Oxides, general purpose primers,
Zinc Phosphates, usually contain anti corrosive pigments.
Galvafroid, Zinc rich paint comparable to hot-dip galvanizing.
Corrosion inhibiting primers give exceptional rust preventing results even on lightly rusted areas.
Primers are generally porous because they lack oil which is what provides the protection from the environment, Synthetic primer should be painted over as soon as possible preferably within a fortnight otherwise it will become too hard and will not allow itself to become part of the paint system.
Aluminium which is described as an oily metal in terms of painting requires etch priming to provide a suitable key to promote paint adhesion.
Acid etch is a corrosion preventive phosphoric acid that can be applied by roller or spray, it is not a substance that can be successfully applied by brush mainly because it is a rapid drying alcohol base formulated to spread thinly and therefore very evenly over the surface.
Aluminium (or Birmabright used on Land Rover) is classed as a reactive metal and like iron it reacts with oxygen causing surface corrosion. Aluminium left in its bare metal state corrodes producing aluminium oxide which is a form of corrosion that will completely cover the surface and yet to some degree protects it from further corrosion. This white aluminium oxide powder formed on bare aluminium can be scraped off, which will lead to further corrosion because the oxide is naturally weak having little or no strength. This process is called anodising.
However anodising aluminium, by an electro chemical process for example, physically alters the metal surface and produces a really tough dense layer of oxide offering maximum protection to bare aluminium.
Sealers or barrier coats are designed to isolate paint substrate reactions from volatile solvents. An example would be if you wanted to spray cellulose over a synthetic base you simply apply a sealer coat first so that when cellulose is applied over the sealer coat because it has isolated the synthetic substrate, so the strong volatile nitro-cellulose thinners will not be able to attack or soften the synthetic substrate.
These sealers are alcohol based so they cannot be undermined by alternative solvent based paints being applied over the top.
You can apply synthetic or oil-based paints over nitro-cellulose or acrylic substrates but you cannot apply cellulose or acrylic paints over synthetic substrate because the solvents will act like paint remover and remove the softer synthetic layers of paint.
If this is the case then an isolation coat is required to form a physical barrier between the old substrate and the new to stop solvent reaction from occurring.
Oil-based paint and varnish are synthetic and not soluble in their own solvent which is turpentine or a similar synthetic thinners, cellulose and acrylic solvent are resoluble.
Iron ore in its natural state does not rust. The rusting process only starts after being wrought when the ore is converted into iron or steel, which is then susceptible to rusting caused by moisture. When the metal is exposed to moisture the rust or ferrous coating, which forms on bare metal as a reddish coating, must be treated immediately to prolong the conversion. Halting or at least slowing down the conversion process is achieved by applying a primer coat initially then subsequent coats of various paints in a bid to improve long term protection from the elements.
However if the surface has become deeply pitted in rust the best way to deal with it is to have the area sandblasted and then use one of the many phosphoric rust converters on the market, which will neutralise rust and keep the metal free from ferrous oxide for a considerable amount of time.
Severely rusted areas that compromise the overall strength of a panel should be cut out and a new section welded in situ. This procedure may be beyond the scope of the average DIY person who is perhaps intent only on providing a vehicle with a repaint without wishing to perform more than just cosmetic paint repairs.
To provide a good base on wood (after rubbing down the surface ready for paint and before any paint is applied) the surface should be wiped over with a solution of boiled linseed oil, goldsize and turpentine or white spirit. The solution should be rubbed well into the grain, impregnating the wood fibres, or until it becomes dispersed and when dry it will prevent the wood grain from raising when further coats are applied on top.
Bringing a wood grain surface up to a first class standard requires more paint material to be applied to fill in the grain, thus building up the surface. It is recommended that an oily primer is used on wood to soak in and ensure adhesion for subsequent coats of paint.
The traditional way of filling the grain of wood was with a knifing filler or brushing filler. Modern methods use a premixed or powdered decorators type filler for wood obtainable at hardware or paint outlets.
Knifing filler or stopper is spread into the grain using a filling knife (usually three inches in width) which is similar to a wallpaper scraper but with a much thinner and flexible blade which allows filler to be spread evenly across the surface.
Polyester filler should be used on metal but it is a harder filler.
After a suitable filling has been applied it is recommended that a stain or guide coat of different colour be added so that when rubbed the high and low areas can be seen. When the surface shows no sign of high or low spots the finish has reached its final rubbing level-this may take several applications.
Rubber rubbing blocks can be used to good effect on flat areas and when rubbing curves a one quarter sheet of wet-or-dry paper can be wrapped around a rag or piece of mutton cloth which will follow curves naturally.
Rubbing in one area too hard or too long or at different pressures should be avoided to eliminate the possibility of rubbing through which will only introduce moisture and cause wood to swell or metal to rust.
When rubbing with a rag or single piece of wet-or-dry your hand must rub in a sideways direction. Rubbing in a forward or backwards direction produces unwanted finger-marks that will show as furrows.
When the rubbing down is finished it should be washed off with clean water and thoroughly dried with a chamois leather and allowed ample time for the moisture to evaporate completely. It is recommended to leave areas that have been wet flatted overnight to thoroughly dry otherwise moisture may cause blistering or peeling to subsequent coats. Initially it is quite practical to rinse bare metal with hot water because this will heat the metal and increase moisture evaporation.
Waterproof rubbing down paper called wet-or-dry is ideal for rubbing as opposed to the old traditional method of pumice stone or cuttlefish bone and it comes in many different grades for almost every surface.
Rubbing down with wet-or-dry paper should be done wet in conjunction with a bucket of clean water with either soda or a small amount of washing up liquid added to lubricate the paper and prevent clogging. Otherwise paint deposits will build up and cause scratching.
During the rubbing process, copious amounts of water must be used to keep the area clean from rubbing slush which, if left to build up, will result in scratching to the surface. The rubbing slush should not be allowed to dry on the surface-it must always be washed off and leathered dry as it contains salts or alkaline. The dried flatting water or sludge can be difficult to remove and may scratch the surface whilst attempting to wash or rub it off. A propriety panel-wipe or pre-wipe is a very rapid-drying degreasing solvent that should be used to clean and inspect flatted areas just prior to paint, removing any contamination (including fingerprints) that can cause adhesion problems.
Panel-wipe can also be used to inspect the surface as it is wiped over wetting a panel which creates a temporary gloss until the spirit dries, usually within a few seconds.
Turpentine can be added to panel-wipe, this slows down the rapid drying so that a surface will remain wet and look glossy long enough to inspect the area.
To obtain the finest quality work it may be necessary to apply filler to certain areas: usually low spots or deep scratches.
Applying a layer of primer will satisfy the porosity of bare wood and discourage suction, generally improving the surface. If the porosity or suction is not satisfied the surface tends to leave a heavy brush mark appearance when painted over which will require even further rubbing to eliminate.
Adding more oil will solve the brush mark appearance slightly but the paint will take longer to dry and remain softer for a longer period.