Varnishing a vehicle is a key process when it comes to obtaining an excellent final finish on a car repainting job.
However, it is also a process in which different problems and difficulties can occur, leading to various painting defects. It is therefore important to take into account a number of factors when varnishing a car that can influence the correct application and final performance of the varnish layer.
1. Correct preparation of the base coats
The first and most important step during the whole varnishing process starts even before applying it: The way we work on the lower coats of paint, will influence the behaviour of the varnish on the part.
A correct prior sanding, polishing or cleaning of the surface are essential aspects to consider achieving an excellent final finish when varnishing a car.
In any of these processes prior to varnishing, make sure that the surface is clean and free of dirt and impurities.
In addition, make sure you do not remove excessive layer micronage, especially during the sanding process, as it may influence the fixing of the varnish. This way you will also prevent the subsequent appearance of circles on the piece or possible painting defects.
2. Mixing ratio of varnish and viscosity
The mixing ratio of the varnish refers to the varnish, catalyst and, in some cases, thinner ratio to be included in the mixture to obtain an ideal product viscosity and application.
viscosity and application. This mixing ratio will always be indicated by the manufacturer on the technical data sheet, so it is essential to always follow the instructions.
This practice should be part of the natural process of preparing the mixture, and it will serve to increase the performance of the varnish in terms of drying time, product flow and final finish.
In order to obtain an exact mixing ratio, we will use the precision scale and other body attachments as regulated cups with different mixing ratios.
An example of a mixing ratio would be 2:1 + 0%-5%, i.e. for every two parts of varnish add one part of catalyst and between 0% and 5% of thinner (depending on the dilution requirements).
3. Varnish temperature and application temperature
Controlling the varnish temperature before it is applied, in addition to that of catalysts and solvents, is a differentiating factor.
Under normal conditions, the varnish should always be at room temperature, around 20-25ºC, although this will vary depending on the type of varnish and the manufacturer’s indications.
Apart from controlling the product’s temperature, the room temperature will also have a direct influence on the finishing results. Extreme environmental temperatures, whether very low or very high, can subsequently lead to painting defects such as fogging, boiling or spraying, among others.
This is the reason why it is advisable to temper the cabin through the different means available (heaters, rays, etc.), taking as a reference a range between 20ºC and 23ºC, as ideal temperatures for the process of applying the varnish.
4. Adjusting the application gun
The spray gun must be set with the pressure and nozzle opening based on the technical data sheet of the varnish to be used. The same data sheet will indicate the values required for a correct application.
The spray gun must be regulated in such a way that the product flow is in the shape of a fan, in order to generate thin, uniform and extended coats.
Note that when varnishing, a larger cloud of atomised product is created than when applying colour, because the pressure exercised during its application is higher.
5. Avoiding excess varnish
Always avoid excess varnish on the part, always trying to apply it evenly.
Some parts are particularly sensitive to excess product, such as vertical parts (flaps, doors, etc.), resulting in possible paint defects, such as sagging varnish.
Excess or lack of varnish on the part will result in a poor finish.
6. Correct cleaning of application equipment
Periodic cleaning of the different paint application equipment such as the compressor, the compressed air line or the spray gun is essential to minimise possible errors and paint defects in the final finish.
A deep cleaning of all the equipment, allows working in a more orderly and safe way, and will be your best ally to prevent defects such as bubbles or silicones.
7. Varnish application method
The varnish must always be applied in a linear way, trying to keep the same speed and distance of the spray gun in relation to the surface during the application at all times. If these parameters vary during the process of applying the coating, the chances of the final finish being defective will increase.
In addition, for large parts, it is preferable to varnish the part by section, avoiding part by part varnishing and product overloading in peripheral areas and edges.
Although it depends on the type of varnish to be used, generally in UHS varnishes (ultra-high solids) a lighter and thinner first coat will be applied to wet the surface.
The second coat will contain a greater load and amount of sprayed product, because it will be applied at a slower speed.
Precautions to take during the painting process
Finally, let us not forget that varnishes are chemical products, and as such there are a number of precautions to follow.
When applying them, use individual protection equipment: mask with chemical filters, nitrile gloves, etc., and the required collective prevention measures: mixing room, application in paint booths or plenums, etc.