As a teenager I, like many of my friends, was keen to modify my car's bodywork as it was too expensive at the time to consider any serious performance enhancements. I was interested to learn some of the refinishing techniques by watching the professionals and gleaning what I could from textbooks and websites. I have since invested in the tools required for me to achieve a professional-looking finish in my home workshop. The following is a fairly thorough (though wordy!) guide to the newcomer to refinishing.
Automotive paint comes in three forms: cellulose, 2-pack (2K) and water-based. I use 2K which is so-called because a two-part (paint+activator) is required. The activator (or hardener) is a polyisocyanate resin that reacts with the paint resins and hardens it by forming an entirely new hard and chemically-stable compound. Little thinner is required with 2K systems so large amounts of paint material can be applied with each spray coat. The polyisocyante content in the hardener cannot be removed by passive filtration so a fresh air supply is paramount. Even though a filter mask can remove all odour from 2K paint, the odourless polyisocyanates cannot be removed and can cause severe respiratory problems or even death.
Cellulose paint contains solid pigment mixed with thinner which evaporates to leave the solids behind. These solids can oxidize over time, turning it milky. The large quantities of thinner used with cellulose paint mean that smaller amounts of solid material are applied with each spray coat, requiring more coats than 2K to achieve a certain thickness. The toxic chemicals in cellulose paint can be removed by a suitable filter mask so it is preferred by some hobbyists but it is no longer used in bodyshops.
I have no experience with with water-based paint. It is more environmentally-friendly as the VOC (Volatile Organic Compound) content is greatly reduced but it does not cure as hard nor last as long as 2K. By law, all European manufacturers use water-based paint on new cars and it is being used increasingly by professional refinishers.
As a relative newcomer to refinishing I have not experimented with many different types of paint. As far as I am aware, the three most popular manufacturers are U-Pol, Nexa Autocolor, and Max Meyer. Their websites contain many useful technical datasheets (TDS). It is my understanding that Nexa Autocolor is preferred by most bodyshops; it is the most expensive but the highest quality. I know of at least one bodyshop that only uses Nexa Autocolor for primers and basecoats and applies clearcoats from one of the cheaper manufacturers. My local paint suppliers can mix any color to order with Nexa Autocolor at around £32 for a half litre which makes a litre when mixed 1:1 with thinner (a full respray takes 3-4 litres). To date, I have used U-Pol primers, Nexa Autocolor basecoat and Max Meyer Lacquer. All 2K paint is compatible regardless of manufacturer.
Paint ancillaries such as thinners and hardeners are sold by many manufacturers including the aforementioned U-Pol, Nexa Autocolor and Max Meyer as well as Omi-Cron and SWS. Paint suppliers will be able to advise. I use 2K thinners for paint mixing and universal cellulose thinners for gun cleaning as it is cheaper. Additional materials include filler, masking tape, masking paper, degreaser and tack cloths. Again paint suppliers will be able to advise. These are described in the Preparation section.
I would estimate the materials cost at around £150 including three litres of primer, half a litre of basecoat, a litre of clearcoat (all unmixed), five litres of cellulose thinner, five litres of 2K thinner and two litres of 2K activator. Most paint should have a shelf-life of two years if kept at room temperature so it's worth buying more primer, clearcoat, thinner and hardener for future jobs.
Spray guns are gravity-fed (paint container above gun), suction-fed (paint container below gun) or pressure-fed (remote paint container). Pressure-fed guns are rarely seen in the home workshop as they are designed to contain very large quantities of paint for refinishing large vehicles. The choice of the remaining two is up to the user; gravity-fed guns contain less paint and place more torque on the wrist but don't waste any paint. Suction fed guns contain more paint and are easier on the wrist but can spit when the fluid level becomes low and may leak when held at steep angles. I prefer to use a gravity-fed gun.
Conventional spray guns require 3-4 bar of pressure at the gun (small in-line gauges can be purchased for this purpose). Growing in popularity are High-Volume, Low-Pressure (HVLP) guns which operate at pressures of around 0.3-0.7 bar. This reduces overspray and therefore saves paint. Though I have never used HVLP guns myself, I have heard that some professionals like to use conventional guns for primer and basecoat and HVLP for the clearcoat.
A middle-of-the-road gun will cost around £30-40 from companies like Machine Mart. A professional DeVilbiss gun can cost in excess of £250 but for the home user I don't believe it's worth spending that much money. A good air compressor will cost £300-400 — buy the most powerful model you can afford and don't buy one with anything less than a 50 litre air receiver.
Spray guns usually have four controls:
I know of professional painters who have used 2K paint for years without any respiratory aid with seemingly no effects. Personally, I'd rather not take the risk! Make note that many toxic airborne chemicals can be absorbed through the eyes so a full-face mask should be used.
There are three options for a breathing air supply. A compressor can be connected directly to a mask via a regulator and a moderate pressure (~8 bar) hose. However, a domestic 2.2 kW (3 HP) compressor can supply around 315 litres/min (11 cubic feet/min) which is shared between a mask, requiring min. 160 litres/min (5.65 cubic feet/min), and a spray gun requiring min. 230 litres/min (8 cubic feet/min). Clearly one compressor is not sufficient in the domestic case.
The second option is to use a compressed air tank. A SCUBA tank is ideal but can be very costly. We tried compressing an empty 19 kg propane tank to 8 bar with my compressor but it could only supply useable air for around six minutes. So we cut the top of one tank and the bottom off the other and welded them together to make a big tank! Probably a bit dangerous, especially as we increased the pressure to 12 bar, 50% more than the standard fill. Here's the proof:
The third is to use a low-pressure high-volume feed from a blower into a mask. This is much quieter than a workshop compressor but requires a wide-bore hose of at least 25 mm (1 in). We now use this as a permament solution using 10 m of 25 mm electrical conduit which is sufficiently flexible and airtight for the pressures involved.