Making your own primer compositions for waterproofing is not just a way to save money, but also an opportunity to obtain a material with ideally selected characteristics for the specific conditions of your facility. Bitumen primer, created by hand, allows you to control viscosity and drying speed, which is critical when working in non-standard temperature conditions or when processing complex surfaces. Ready-made factory emulsions often have an average composition, which may not be suitable for porous concrete or rusty metal without preliminary modification.
The process of mixing components requires strict adherence to safety precautions and accurate calculation of proportions, since the durability of the entire roofing system or foundation depends on the quality of adhesion. In this article we will analyze the chemical basis of the interaction of bitumen with solvents, consider the necessary tools and provide time-tested recipes that professional roofers use. You will learn how to avoid common mistakes that lead to coating peeling or cracking after a short time.
Purpose and principle of operation of the primer composition
Main function primer consists of binding dust and filling the micropores of the base, creating a continuous hydrophobic layer. When you apply the mixture to concrete or cement-sand screed, the liquid fraction penetrates deep into the structure of the material, and after the solvent evaporates, a durable bitumen film remains. This film acts like glue, providing a monolithic adhesion to the base waterproofing carpet with the surface, preventing swelling and ruptures under the influence of temperature deformations.
The use of a homemade composition is especially important when it is necessary to treat large areas at minimal cost, or when it is necessary to obtain a composition with a non-standard viscosity. For example, for the initial treatment of a heavily dusty base, a more liquid mixture is required than for application to smooth metal. Correctly selected solvent allows you to adjust the life time of the mixture, giving the master the opportunity to work at a comfortable pace without haste.
⚠️ Attention: Never use a water-based primer over a fresh bitumen layer without waiting for the moisture to completely evaporate, as this will cause the water to boil and cause bubbles to form in the waterproofing.
In addition, the primer acts as an anti-corrosion protection for metal roofing elements and pipes. Bitumen resin isolates the metal from contact with oxygen and water, stopping oxidation processes. If you plan to use weld-on materials, then the presence of a high-quality primer is a mandatory requirement for roll coating manufacturers to maintain the product warranty.
Selection of components: bitumen and solvents
The foundation of any primer is bitumen binder, which is most often purchased in solid form (BN 90/10, BN 95/5) or leftovers from other work are used. Solid bitumen requires pre-melting, which creates additional difficulties with heating and the risk of fire, but it allows you to get the most predictable result in terms of heat resistance. Liquid bitumen mastics already contain solvents, but their use to create a primer may not be economically viable due to the high cost of the finished product.
The solvent most often chosen is gasoline "Galosha", diesel fuel or kerosene. The choice depends on the required drying speed and working conditions: gasoline evaporates very quickly, which is convenient in hot weather, but requires prompt work. Diesel fuel takes longer to dry, penetrates deeper into the pores of concrete, but leaves a specific odor and requires more time to gain strength before fusing the base layer.
It is important to consider that adding motor oil in the composition (about 5-10% by weight of bitumen) gives elasticity to the frozen layer, preventing it from cracking in the cold. However, excess oil can cause the primer to remain sticky and not dry completely, which is absolutely unacceptable for high-quality waterproofing. Proportions must be selected experimentally for each batch of raw materials, since the composition of oil and processing technologies is constantly changing.
Necessary tools and safety precautions
The cooking process involves the use of flammable liquids and high temperatures, so preparing the workplace and protective equipment comes to the fore. You will need a metal container (bucket or barrel) with thick walls that can withstand direct fire or a powerful heating element. The use of plastic containers even for mixing cold components is unacceptable, since aggressive solvents can instantly melt the container.
To mix the thick mass, you will need a strong metal stirrer or wooden mallet, as well as a fire source (fire, gas burner) in case of hot cooking. If you use the cold dissolution method, an airtight container and time to infuse will be enough. Do not forget that solvent vapors are heavier than air and accumulate in low areas, so it is better to carry out work outdoors or indoors with powerful forced exhaust.
☑️ Safety checklist
Personal protective equipment should include a respirator, since inhaling gasoline or solvent vapors causes severe poisoning and dizziness. Clothing should cover all areas of the skin, and shoes should be resistant to chemicals. There should always be sand or a fire extinguisher in the immediate vicinity of the cooking site, since spilled and ignited bitumen cannot be extinguished with water - this will lead to splashing of the burning mass.
Cooking technology: hot and cold methods
There are two main ways to prepare primer: hot (cooking) and cold (dissolving). The hot method is considered more traditional and allows the use of bitumen of any hardness, turning it into a homogeneous mass. First, solid bitumen is melted in a container, heating it to a temperature of 160-180 degrees Celsius, constantly stirring and skimming off the foam. After obtaining a homogeneous liquid, remove the container from the heat and allow it to cool to 80-90 degrees before starting to introduce the solvent.
The cold method involves grinding solid bitumen into small pieces and filling them with solvent in an airtight container. This process takes from 2 to 7 days depending on the ambient temperature and the size of the bitumen pieces. The mixture must be shaken periodically. This method is safer, since the risk of fire is eliminated, but it requires advance planning of the work, since you will not get an instant result.
⚠️ Attention: Pouring solvent into hot bitumen is strictly prohibited - this will cause instant ignition of vapors and explosive combustion.
With the hot method, the solvent is added in a thin stream to the cooling bitumen, vigorously stirring the mixture. If you pour in the solvent quickly, the top layer will flare up, but the bottom layer will remain thick. Temperature here it is critical: bitumen that is too hot when in contact with gasoline can cause a fire, and bitumen that is too cold will not mix evenly, forming lumps that will then clog the sprayer or create unevenness on the surface.
Proportions and composition table
Exact adherence to the recipe is the key to obtaining a material with the required viscosity. Different types of work require different ratios of bitumen and solvent. For example, for the primary primer of porous concrete, the mixture must be thinner to penetrate the structure, whereas for metal, a more concentrated composition can be used. The universal proportion is considered to be 1:3 or 1:4 (one part bitumen to three to four parts solvent), but it can vary.
Below is a table with recommended proportions for various operating conditions and types of substrate. These values are starting values, and depending on the specific brand of bitumen and air temperature, the amount of solvent may need to be adjusted.
| Base type | Purpose of application | Proportion (Bitumen: Solvent) | Drying time |
|---|---|---|---|
| Concrete screed | Deep impregnation | 1 : 4 | 2-4 hours |
| Metal | Anti-corrosion | 1 : 3 | 6-12 hours |
| tree | Waterproofing | 1 : 2.5 | 12-24 hours |
| Ruberoid (repair) | Bonding | 1 : 3 | 4-6 hours |
When preparing a large volume of mixture, it is recommended to first make a test batch in a small amount to check drying speed and adhesion. The optimal viscosity of the finished primer should resemble liquid sour cream or motor oilso that it spreads easily, but does not instantly drain from vertical surfaces. If the mixture is too thick, it will not fill microcracks; if it is too thin, it will not create a sufficient bonding layer.
Application and typical mistakes
The prepared composition can be applied with a brush, roller or spraying method, if viscosity allows. The surface must be dry and free of dirt, oils and loose elements. Before starting work, it is advisable to heat the base with a burner to remove any remaining moisture, since water is the main enemy of bitumen waterproofing. The primer should be applied in one layer, thoroughly rubbing it into the surface.
What to do if it rains?
If it rains immediately after application, the layer will most likely be damaged and will have to be redone after it has completely dried. If the primer has already set (more than 2-3 hours have passed), it can withstand short-term exposure to water, but the quality of adhesion will decrease.
A common mistake is to apply primer on a damp substrate or in conditions of high air humidity (fog, dew). In this case, water remains under the bitumen film, which, when heated by the sun, turns into steam and peels off the waterproofing with bubbles. Also, you should not prepare the mixture “for future use” for several days in advance, since the solvent evaporates and the composition thickens, losing its penetrating properties.
To check that the base is ready for fusing, press a gloved finger onto the dried primer: if it does not stick or get dirty, you can begin laying the base layer.
Another problem is uneven mixing, when a thick sediment remains at the bottom of the container, and pure solvent floats on top. Before each use, even during operation, the mixture must be thoroughly mixed. Ignoring this rule will result in you applying a weak solution at the beginning of the work, and pure bitumen at the end, which will disrupt the uniformity of the waterproofing carpet.
The quality of waterproofing depends 80% on the preparation of the base and the correct application of the primer, and not just on the cost of roll materials.
Questions and answers (FAQ)
Can used motor oil be used instead of solvent?
The waste can only be used as an additive (no more than 10-15%) for elasticity, but not as the main solvent. The waste contains combustion products and metal impurities, which reduce adhesion and can cause a chemical reaction with bitumen. Completely replacing gasoline or diesel with exhaust will make the mixture too oily and it may never dry out.
How long does it take for primer to dry at +5°C?
At a temperature of +5°C, the process of polymerization and evaporation of the solvent slows down significantly. If at +20°C the primer dries for 2-4 hours, then at +5°C this time increases to 12-24 hours or more. Working at such temperatures is not recommended, as condensation on cold concrete can damage the adhesion.
How to dilute thickened primer?
If the mixture has thickened due to the evaporation of volatile fractions, you can add the same solvent that was originally used (gasoline, kerosene or diesel). You need to add small portions with constant stirring until the desired consistency is achieved. Water is absolutely not suitable for these purposes.
Can primer be applied to wet concrete?
Absolutely not. Bitumen does not allow water to pass through, and if you apply it to a damp base, the moisture will remain inside. When heated by the sun, it will turn into steam, create pressure and tear off the waterproofing carpet, forming blisters. The base must be absolutely dry.