A Practical Guide to Vehicle Refinishing

Julian Woodstock

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781351040525
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Aviación

A guide to refinishing suitable for complete beginners and more advanced technicians.

This heavily illustrated guide will help students through their Level 2 and 3 vehicle refinishing qualifications and be useful as a reference and trouble shooter for more advanced technicians. It is set out in the order in which a vehicle is repaired.

There are sections covering:

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• identifying different substrates, with an explanation of how this affects the materials to be chosen and techniques to be used

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• preparation work required prior to the application of foundation materials

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• how to choose the correct foundation material

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• shaping and sanding techniques

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• different types of popular top coats and the required application techniques

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• glossaries for tools and equipment

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• health and safety considerations

This book has been designed and written by a true 'petrol head' whose career and hobbies have revolved around motor vehicles and the refinishing trade. He has been in the motor trade for more than 16 years and has delivered Refinishing qualifications to students for over 11 years. Someone who has never before held a spray gun should be able to understand stage-by-stage, or they can dip in for precise trouble shooting and tips.

Chapter 1

Identifying substrates

One of the first tasks to consider when undertaking any form of paint repair is to identify the substrate. This is very important because the substrate may need to be protected by the paint and may require special undercoats to provide adhesion or corrosion protection. Some substrates can react with top coats, whereas others can be damaged by cleaning agents or undercoats.

So what is a substrate? Well, substrates can be broken down into two main categories: parent substrates and original or repainted panel coatings.

1.1 PARENT SUBSTRATES

These substrates are the material that the panel or component is made from. For example, the majority of motor vehicle body panels are made from low carbon (or mild) steel. However, many other materials may be used to manufacture body panels including plastics, aluminium, composites and alloys.

Low carbon steel will require an undercoat with some form of anticorrosion material, such as an acid etch primer. The easiest way to identify a steel component is to use a magnet. Only ferrous metals will attract a magnet. Of the ferrous metals available, only low carbon steel and its similar alloys are used in body construction. Stainless steel has been used very occasionally (the DMC DeLorean most famously), but it is usually nonmagnetic. If there is rust present on the panel, like the one in Figure 1.1 this will prove that the panel must be made from low carbon steel and is about the only visual clue to be relied upon.

If the panel concerned is metal but nonmagnetic, it is most likely to be aluminium. This also requires an etch primer, but for the reason that aluminium naturally builds a protective oxide layer very quickly. In reality, this oxide layer is not well attached at a molecular level, and although paint can ‘stick’ to it well, the oxide layer will peel from the metal easily causing adhesion problems. Therefore, bare aluminium should be thoroughly abraded and cleaned with a solvent panel wipe, then an etch primer applied immediately. A simple visual check for corrosion, which appears as a white powdery substance, as pictured in Figure 1.2 can help to identify aluminium.

Figure 1.1 This panel is obviously very rusty, however the metal has not been made thinner by the corrosion. There is clean silver steel visible where the rust has been removed. This will then be treated with a rust converter before being primed and painted.

Figure 1.2 This alloy wheel has severe corrosion. The white powder is clearly visible on the bare metal areas.

If the panel or component is obviously made of plastic, it is most likely to be a thermoplastic. Thermoplastics are plastics that can be warmed and reshaped (a useful benefit if the panel is dented or deformed). They are most easily identified by a code embossed on an unseen part of the component such as PP + EPDM (polypropylene and ethylene propylene diene monomer) as in Figure 1.3. If a code cannot be found, the information can usually be found in data manuals. Of course, if either of these is not forthcoming, an unseen part of the component can be warmed and attempts made to shape it, although this should be a last resort.

Figure 1.3 This plastic code was found on a Renault wing.

Plastic can be visually identified by sanding through the paint layers. Thermoplastics used in vehicle body components are usually black as in Figure 1.4, white, or grey; therefore, if such a substrate is found, it is likely to be a thermoplastic. These components can often be more flexible than metal components, so plasticiser (or elasticiser) should be added to the paint. More importantly though, the bare plastic should not be cleaned with a solvent panel wipe (water based should be used) as the solvent can be absorbed by the plastic and may show itself as Solvent Pop in the paint finish after baking. Finally, and possibly most importantly, bare thermoplastic requires the application of an adhesion promotor to ensure that any undercoats applied over the repairs remain stuck to the component.

Thermoset plastics and composites are not used as frequently as thermoplastics, and they can be identified in a number of different ways. As has already been discussed, thermoplastics can be warmed and reshaped. It should therefore be obvious that thermoset plastics cannot be reshaped in such a way; they are indeed often very rigid. Composites are most easily identified in a non-painted state; therefore, if it is possible to view the reverse side, this will help with the identification. Composites in body construction are usually made of strands such as glass fibre or, more rarely and exotically, carbon fibre. Glass-reinforced plastic is most often made from a randomly arranged matting of glass fibre as can be seen in Figure 1.5 whereas carbon fibre is structured as a woven material. Both are set in resin, which is hard when cured. As a result, they can be identified when sanding by a strong and distinct odour, although obviously a dust mask should be worn for all sanding operations. Care should be taken when sanding composites not to go through any gel coats that are present. Like thermoplastics, thermosets and composites should not be cleaned with a solvent panel wipe as it may evaporate through drying primer coats.

Figure 1.4 Here the white paint has been sanded back on this bumper to expose the black plastic. Note the pinky beige patch of body filler which has been applied to a damaged area.

Figure 1.5 This panel was found in the front of a VW Beetle. The distinct random fibre on the reverse makes it easy to identify as glass reinforced plastic.

There are other parent substrates; however, the ones covered here are the most commonly found. Materials such as wood, tin and even fabric have been used in vehicle body construction over the years, but they are so rarely found we won’t go over them here. The following are panel coatings that should be considered as substrates when being over coated.

1.2 ORIGINAL EQUIPMENT MANUFACTURER (OEM) PAINT MATERIAL

This is what the panel was originally painted in at the factory. The age of the vehicle should be considered, as there are a huge range of paint materials that could have been applied to the panels at the factory – different periods of production will indicate the most likely used material. These may include single-pack (1K) or two-pack (2K), high-bake enamels and ceramic ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Acknowledgements
• Author
• Introduction: The logical sequence to refinishing
• 1 Identifying substrates
• 2 Identifying the damage
• 3 Application of foundation materials
• 4 Preparation of panels for top coats
• 5 Masking of vehicle and components
• 6 Paint application equipment
• 7 Application of top coats
• 8 Health and safety in the refinishing workshop
• 9 Tool and equipment maintenance
• 10 Glossary
• Index