Says Steve Louis, Sales & Marketing Manager, HMG Transport Coatings Division When spraying large vehicles, size clearly does matter, but so do many other factors that should be taken into consideration in the pre-planning of any paint job. Large vehicles are usually complex shapes and they usually involve different substrates, multiple colour schemes to achieve corporate livery standards and, complicating matters even further, a variety of application equipment and operator conditions; spraying and curing facilities, as well as drying temperatures, also play a major role in the planning stage.

When considering a large paint job, it is also important to understand the consequences of your decisions; for instance, choosing a fast hardener and thinner may give you quick drying times, but the downside is poor wet edge times and, in consequence, a dry dull finish. The primary objective is always to achieve the best aesthetic finish in a reasonable amount of time, within the confines of a labour and materials budget, bearing in mind that rework on such large vehicles is an absolute no-no.

Technical paint knowledge is sometimes no substitute for practical experience either. What is in theory the best system to use, may not turn out to be the case in practice. Our customers prove that to us time and time again; after all, they are the expert applicators. Here then are some of the key points to consider, when spraying large vehicles:

Colour Matching
First, always take time to check that you have either mixed up, or been supplied with, the correct colour. Spray out a test panel first, then check it against an approved standard or have the vehicle owner approve it, since repainting a vehicle due to a poor colour match is a very expensive pastime.

Application Equipment
Unlike car refinish work, commercial vehicle refinishers have a whole range of application equipment to choose from, including conventional hand-held gravity or suction HVLP, pressure-pot HVLP, diaphragm pumps, air-assisted, airless and electrostatic technology. Each has its own advantages, according to the substrate type, size, shape, material to be used and desired finish, so equipment manufacturers should be consulted in order to maximise potential. In my opinion, the industry sometimes fails to take full advantage of this free expertise, especially since choosing the correct equipment can make the difference between poor, good and absolutely first-class paint jobs.

Hardener / Thinner
The choice of hardener/thinner is one of the most critical factors and needs to be made based on vehicle size, spraying temperature, application equipment and the number of operators likely to be spraying at the same time. It is always best to use the specific hardener and thinners recommended by the paint manufacturer, since these have been developed together in order to offer the best performance balance and will work in complete harmony with one another.

Curing Times
At HMG Transport Coatings, we have a range of hardeners and thinners to suit different temperatures and, if in doubt, we always suggest opting for the slowest system available, when spraying large vehicles. Contrary to common belief, this will not slow down through-cure and will give you plenty of wet edge time, leading to ease of application and optimum finish. Spraying large vehicles tests one's fitness enough, without having to race around like Linford Christie to maintain a wet edge, so let the system work for you. What's more, through-cure and finish are often at their best when using slower systems.

Topcoat System
By far the best topcoat choice for refinishers and fleet owners are high performance 2K acrylic systems. Although slightly more expensive than the traditional alkyd polyurethanes used historically, their workability, drying times and overall performance make these 2K products the new industry standard. Moreover, with the latest environmental legislation specifying the use of a topcoat system with no more than 420g/l of VOC, this is probably the easiest decision to make, as it's effectively made for you.

Multiple Colours
Dependent on the colour scheme, you could use either a solid direct gloss or a solid basecoat with a clearcoat over the top, even a combination of the two systems. Whilst the clear-over-base system has long been popular, the advent of waterborne basecoats means the industry is facing new challenges in perfecting the application of this new technology. Either way, remember that the ancillary products available for each refinishing system are there to assist you in achieving the optimum finish, in the fastest process times.

Masking Products
Masking products are very important whichever system is employed, particularly when applying multi-colour liveries. The use of incorrect tapes, papers or polythene sheeting can damage fresh paintwork beyond recovery, resulting in expensive rework, so don't cut corners in this area.

Drying Times
Remember, the drying times presented on technical data sheets are theoretical and are typically based upon metal temperatures. So a key point to consider is whether the vehicle is constructed from steel, aluminium, GRP or another form of plastic, as each will absorb heat differently. Twenty-gauge steel will heat up quicker than a big chassis rail, for example, and will absorb more heat than GRP or plastic and retain temperatures for different lengths of time. Also, whilst the data sheet may advise baking at 70ºC for 30 mins, it may be half an hour before the vehicle is actually up to temperature, so that has to be taken into account too.

Finally, when spraying large vehicles, it is always best to take time at the beginning of the job to plan the whole process. Just because it's the same type of vehicle you painted last week, are the circumstances exactly the same? Is it one colour or multiple, how many staff are available, which primer system is best and what value is involved in the job? Then ask yourself how well did that last job go and can your work be improved upon? Also, don't overlook the advice and expertise freely available from your suppliers, in terms of paint selection, prep materials, application equipment, training and best practices, all of which will help you produce the perfect finish.

After all, size isn't everything, it's the complete package that counts.