TYPES OF PUTTIES. WHICH PUTTY TO CHOOSE?
TYPES OF PUTTIES. WHICH PUTTY TO CHOOSE?
Which putty to choose? Previously, such questions simply did not arise. Which one? Naturally, the one that is available! However, nowadays, in the assortment of repair material manufacturers, there is a good dozen of putties, each possessing entirely different properties. How can one navigate through this variety?
BACKGROUND
Putties began to be widely used in the late 1950s. Prior to that, low-melting point solders were used for dent removal and surface leveling. Initially, the details were hammered and shaped using an underlying stamp (remember that in the past, body panels were much thicker). Damages were filled with a melted solder until it reached a malleable state and then, through the use of special files, the final shape was formed. This process was laborious, low-tech, and time-consuming, which is entirely out of sync with our times. Moreover, working with open flames and molten solder in a workshop, full of lead dust, posed health risks to workers. While such concerns may not have received much attention in the 1950s, modern times have placed greater emphasis on safety and labor protection requirements. It is no wonder that a paste-like product, which can be quickly and easily mixed, applied, and shaped without heating and special equipment, soon gained popularity.
WHAT IS PUTTY?
Putty is a product with a paste-like or liquid consistency that is applied to remedy minor surface defects before painting.
Any putty consists of a mixture of fillers with pigments (the former responsible for strength, ease of processing, settling properties, elasticity, while the latter for color) in a binding substance (responsible for surface adhesion, polymerization speed, settling properties, strength, and elasticity) with various additives (plasticizers, stabilizers, etc.).
In our time, polyester putties (PE) have gained widespread use in automotive body repair due to their chemical base in the binding substance. Compared to acrylic and nitro-based putties, polyester putties are the least susceptible to settling (when applied in relatively thick layers) and exhibit maximum polymerization speed. However, many professionals are not even aware of the existence of other putties, such is the current popularity of polyester-based fillers.
Note: In the context of automotive body repair, the term “putty” is often used interchangeably with “filler.”
TYPES OF POLYESTER PUTTIES
The process of surface leveling consists of two stages: filling and shaping. Initially, preliminary leveling is performed, addressing deep irregularities, followed by final surface shaping.
Based on this principle, all putties can be divided into two main groups:
- Filling Putties
- Finishing Putties
Filling putties quickly address deep irregularities. However, most of these putties have a coarse-grained structure and cannot create a smooth, non-porous surface (e.g., fiberglass putty). As a result, they require further application of finishing putties.
Finishing putties are fine-grained materials. They are elastic, easy to apply, and can be easily sanded. On one hand, they are used as decorative layers after coarse-grained putties, and on the other hand, they serve as standalone products for addressing minor defects.
There are also universal putties that possess properties of both filling and finishing putties simultaneously. Different materials are applied for surfaces made of metal and plastic. Putties can be applied using two methods: high-viscosity ones applied with a putty knife and liquid ones applied with a brush or spray gun. To better understand which putty is best suited for various types of work, let’s explore in more detail the properties of the main types of putties offered by leading manufacturers of repair materials.
Glass Fiber Reinforced Filling Putties
When it is necessary to restore corroded or severely deformed parts, glass fiber reinforced putty is used. Additives of chopped glass fiber provide this putty with high strength characteristics, reinforcing properties, and excellent filling ability. It can be applied in thick layers, quickly concealing even deep irregularities. This type of putty is also used for repairing various through damages in parts and metal tears. Glass fibers act as a kind of bridges, preventing material from sagging over hollow spaces.
Although this putty is strong, it is not recommended for use on large surfaces, such as the central part of the roof, as it lacks elasticity and may develop cracks under vibration. Ideal areas for application include thresholds, pillars, arches, and other structural elements of the vehicle body.
Aluminum Reinforced Filling Putties
To address defects on surfaces exposed to temperature fluctuations and vibrations, such as the roof, hood, and engine compartment, it is recommended to use aluminum-reinforced putty. Thanks to the aluminum powder filler, this putty exhibits a coefficient of thermal expansion similar to metal, allowing it to endure repeated heating and cooling cycles without cracking or delamination.
Despite being categorized as a “filling” putty, it has a fine-grained structure and can also be used to address minor defects. It achieves a smooth finish without chipping at the joints, which is common for medium and coarse-grained materials. Additionally, this putty is the least susceptible to settling: aluminum is incapable of saturating a large amount of solvent (whose evaporation causes “sinking”). Among its other advantages is a longer working time compared to others—up to 8 minutes (others up to 5 minutes)—which is beneficial when working on large surfaces.
TYPES OF LIGHTWEIGHT FILLER PUTTIES (LEICHT, LIGHT, ALLOYED)
Many have probably experienced the situation when lifting the car hood feels like a weightlifting exercise. In such cases, when dealing with extensive repair work, and the increase in weight should not be noticeable, lightweight putty is recommended. Such putties are lighter than regular ones by a significant 30-40%!
The low specific weight is achieved through a special filler, often microscopic hollow glass beads. Besides the reduced weight, these putties also offer ease of application, as the sanding process is not about “pulling out” the filler from the resin, but rather breaking the glass beads. Additionally, this putty does not “stone harden” and remains equally easy to work with both 30 minutes and 3 days after application.
All these features make lightweight putty an ideal material for working on large areas, such as car panels, buses, and wagons, where rapid sanding is required.
CLASSIC FILLER PUTTIES
“Regular” filler putties can serve as the main material for surface leveling. They exhibit excellent filling properties and sand smoothly, making them ideal for overall surface leveling and subsequent finishing using a plane.
When dealing with very deep irregularities, this putty can be applied over fiberglass without the need for prior sanding (fiberglass is generally not recommended to be sanded as dust may enter the respiratory system). These are often medium-grained materials, which should be overlaid with a fine-grained finishing putty to achieve the best quality.
FINISHING PUTTIES (FEIN, FINISH)
Finishing putties have a creamy consistency and fine structure, making them easy to apply without leaving pores. They are elastic and sand excellently, serving as decorative layers after applying coarse-grained materials (fiberglass, lightweight putty), as well as standalone materials for addressing minor surface defects (scratches, small dents).
Finishing putties have a white color, making it easier to inspect surface quality during dry development while sanding.
UNIVERSAL PUTTIES (MULTI, UNI, UNIVERSAL)
The name speaks for itself. Universal putty is a versatile product with a wide range of applications. Firstly, they have excellent adhesion to all types of surfaces encountered in body repair, including aluminum, galvanized, steel, plastic, and fiberglass (hence the name).
Secondly, they can be applied in both relatively thick and thin layers. After polymerization, they maintain high elasticity and sand smoothly.
LIQUID PUTTIES (SPRAY)
Liquid putty can be an effective solution for addressing defects on large surface areas. With a high content of dry residue, it possesses good filling properties, quickly addressing all minor surface defects. It dries rapidly, guaranteeing a completely non-porous surface. It sands easily.
However, it is essential to remember that despite the application method, this putty remains putty and requires covering with a primer-filler.
PLASTIC-SPECIFIC PUTTIES (FLEX, PLASTIC, BUMPER FIX)
To address defects on plastic surfaces (bumpers, spoilers, areas of plastic element fusion and bonding), special putties for plastic are used. They are based on a mixture of resins with shredded rubber and plastic particles, providing exceptional elasticity and adhesion to plastic parts.
From the above, we can conclude that an auto repair specialist should have at least three types of putties in their inventory: one filling, one finishing, and one for plastic.
PUTTY COMPOSITION AND SETTLING PROBLEM
Polyester putties consist of resins based on modified unsaturated polyesters and fillers. To achieve the putty’s workable consistency, the mixture of these components is dissolved with styrene. As a result, all consumer properties of a particular putty depend on the quality and properties of the resin and filler, as well as the amount of solvent.
Resin: The resin is the primary component that determines the quality of the putty. It is responsible for adhesion to the repaired surface, drying time, strength, and ease of application.
The highest quality resins are considered products from chemical giants like Bayer and BASF. If these manufacturers’ raw materials were used in the putty production, you can be assured of the high quality of these repair materials.
Primarily, fillers are solid inert substances of a certain size that do not dissolve in the overall mass. Since the creation of polyester putties, traditionally used fillers have included finely ground powders of minerals such as chalk, talc, asbestos (now banned), kaolinite, and gypsum. Later, wollastonite and bentonite were introduced, and in recent years, quartz microspheres and phenolic microspheres have become popular.
The size and properties of filler particles directly influence the properties and purposes of the putties. For example, aluminum particles improve the material’s thermal conductivity, fiberglass additives provide reinforcing properties, and glass microgranules offer a low specific weight.
Solvent: Styrene, which is part of the polyester resin (highly hazardous to human health) and participates in the polymerization reaction, partially evaporates during the process, making it one of the main elements affecting one of the most crucial putty characteristics – settling (volume reduction during polymerization).
An optimal amount of solvent in putty is considered to be 30% of its volume. Putties with a higher solvent content are cheaper but more prone to settling. As the putty dries, the solvent evaporates, reducing its volume. The more volatile fractions evaporate, the more noticeable the defect becomes.
For this reason, the thickness of putty layers should be minimal, determined by the material’s settling rate. Quality putties typically settle by approximately 2%. Thus, the putty should be applied in layers of such thickness that a 2% reduction in volume from this amount is not visibly noticeable to the naked eye.
Settling of the material and the appearance of the puttied area can also be influenced by other factors, such as the method of treating the old surface before puttying. For example, if the boundary between metal and the old coating is smooth, settling in this area will be only 2 microns (2% of 100 microns). However, in case of abrupt disconnection, settling will be as much as 20 microns (2% of 1 mm).
The application method can also influence the settling of putty. For instance, if insufficient pressure is applied while applying a layer, voids will inevitably be left in deep areas, where the putty will sink later.
qually important is the secondary settling that occurs as a result of the influence of solvents from applied materials (liquid putty, filler primer) on the surface of the putty. This phenomenon is characteristic of inexpensive putties, where low-quality resins and fillers allow solvents to “eat away” the top layer and penetrate deep into the putty layer. Subsequently, during the drying process, these solvents evaporate, leading to a gradual compaction of the putty.
This problem can also be aggravated by poor-quality primers with low dry residue content and aggressive solvents in their composition. Thus, the negative effect becomes twofold: the subpar primer corrodes the putty layer and causes settling, while the low filling capacity of the primer prevents the leveling of this defect through sanding.
The conclusion here can be straightforward: it is essential to strictly adhere to the technology and prefer high-quality proven materials. There is one simple way to verify this: putty made from quality raw materials cannot be cheap.
RESUME
For standard repairs, it is recommended to use filling putties, with coarse and medium-grain materials to be covered with fine-grain finishing putty.
Fiberglass-reinforced putty is recommended for filling deep irregularities, holes, metal tears, and through corrosion. It is not advisable to apply it to body parts subjected to strong vibration (e.g., the roof). Ideally, it suits thresholds, arches, and pillars.
In areas exposed to vibration and temperature fluctuations (hoods, roofs), it is recommended to use putty reinforced with aluminum.
For tasks with substantial work volume where weight increase should not be noticeable, lightweight putties are recommended.
Liquid putty is applied with a sprayer and is used as a finishing putty for large surfaces.
For repairing plastic components (bumpers, spoilers), plastic putty is employed. Finishing putties with fine grains can also be used to address minor scratches and chips on plastic elements.
For work on aluminum and galvanized surfaces, we recommend using universal putty, ensuring high adhesion. Other putties may only be used on such surfaces if indicated in the technical documentation or if the surface has been previously treated with epoxy primer.
