How to paint the screw of the outboard motor: the choice of composition and technology

The propeller is a transmission element that takes on enormous loads during the operation of the outboard motor. Constant contact with water, impacts on floating objects, exposure to sand suspension and aggressive chemical environment make it one of the most vulnerable parts, which is why the question of how to paint the propeller of the outboard motor worries not only aesthetes, but also pragmatic owners of equipment who want to extend the life of the unit.

The main purpose of painting in this case is not so much to make the product look good, but to create a reliable barrier against corrosion, which can destroy the hub and blades from the inside. In addition, the right coating helps to reduce the fouling of algae and shells, which is especially true for motors that often remain in water. In this article, we will discuss the physical and chemical requirements for paint materials and choose the optimal solution.

It should be noted that the use of conventional nitro-emali or automotive paint "metallic" will not bring the desired result. The specifics of the screw require materials that have high adhesion to non-ferrous metals and, most importantly, exceptional elasticity, without this property, the coating will quickly be covered by a network of microcracks under the influence of hydrodynamic loads.

Coverage requirements under cavitation

The propeller is accompanied by the phenomenon of cavitation, where water boils in pressure-dilution zones, forming vapor bubbles, and these bubbles collapse at a tremendous rate and create a shock wave that literally rips out microparticles of metal. If you decide to paint the screw with the wrong paint, cavitation erosion will destroy the coating in a matter of hours of operation of the motor.

The coating material must be high mechanically Hard but fragile enamels, such as some epoxy resins without plasticizers, can simply detach from the layers, while too soft compounds will be washed away by the flow of water, and the ideal balance is achieved by using specialized anti-corrosion enamels for the underwater part of the body or transmission paints.

⚠️ Attention: It is strictly forbidden to use water-based paints (acrylic, latex) for painting the screw. They will not withstand contact with water under pressure and instantly lose their protective properties, without providing any protection against corrosion.

In addition to mechanical resistance, it is critically important. chemical inertiaWater in water bodies can be fresh, salty or contain aggressive impurities from industrial effluents. The paint must not react with electrolytes, otherwise there will be a galvanic vapor, which will accelerate the corrosion of the screw itself, especially if it is made of aluminum alloy and the hub is made of steel.

Overview of paint types for the underwater part

The paint market offers several product classes that are theoretically or practically applicable to propeller protection, and understanding the difference between them will avoid fatal mistakes in choosing from the first place, you should pay attention to specialized formulations developed for marine applications.

One of the most popular groups is alkyd-urethane enamelThey form a strong, glossy film that is resistant to abrasion. But their main enemy is the impact load. If your screw often comes into contact with the bottom or hard objects, such paint can chip, but for quiet operation in clean water bodies, this is a reasonable budget option.

A more advanced solution is considered epoxy They provide what's called "cathode protection," sacrificing themselves to preserve the base metal, often used to paint the lower units and legs of the motor, so they're as compatible as possible with the propeller materials.

Antifouling paints deserve special attention, but here you need to be extremely careful. Many of them contain biocides (for example, copper compounds), which are toxic to aquatic flora and fauna. forbiddenCopper causes instant and deep corrosion of aluminum through a galvanic reaction.

• 🎨 Two-component polyurethanes: They form a super-strong coating resistant to gasoline, oil and salt water, but require ideal surface preparation and are difficult to apply.
• 🛡️ Zinc-containing soils: Provide better anti-corrosion protection due to the effect of the tread anode, ideal for steel elements.
• 🌊 Silicone antifouling: Create a slippery surface to which algae stick poorly, but have low mechanical strength and are easily damaged.

Surface Preparation: A Key Stage

Even the most expensive and high-quality paint will not stay on a poorly prepared surface. In conditions of constant vibrations and hydrodynamic loads, adhesion plays a crucial role, the preparation process takes up to 70% of the entire work time and requires carefulness.

The first step is the complete screw-cleaning It's from old coatings, oxides, oil stains and biological fouling, and you can use mechanical brushes, scrapers or chemical washes for old varnishes, and it's important to get to the pure metal, especially in the places where there's chipping and corrosion, which is often hidden under a layer of old paint.

After mechanical cleaning, you need to carry out degreasingUse acetone, white spirits, or specialized degreasers. Rub the surface with a rag soaked in a solvent and wait for it to evaporate completely. Any residue of oil, lubricant, or silicone will cause the paint to lie unevenly and soon start peeling off.

If deep corrosion shells or mechanical damage (snacks) are found on the surface of the screw, they must be splashed, special epoxy putty devices are used to hold the metal underwater, and after drying, the putty is grinded level with the main surface, creating a smooth profile of the blade.

⚠️ Attention: When grinding aluminum screws, do not use steel wire or iron brushes. Iron particles embedded in aluminum will become hotbeds of spot corrosion, which will quickly destroy the metal under the paint.

Technology of applying paint material

The process of painting the screw is not much different from painting other parts of the outboard motor, but has its own nuances. Work should be carried out in a dry, well-ventilated room at an air temperature of 15 to 25 degrees Celsius. Humidity should not exceed 80%, otherwise moisture on the surface can condense, which will worsen adhesion.

Applying paint is best by method spraying This allows you to create a thin, uniform layer without leaks or flow, and the brush or roller can leave grooves that create additional resistance to water and are the starting points for coating detachment. If you use a brush, the movements must be light and fast.

The optimal painting pattern involves applying one layer of soil and two layers of finish enamel. Each layer is applied only after the previous one has completely dried. The drying time is specified by the manufacturer on the package and depends on the ambient temperature. Do not try to speed up the drying process with a hairdryer or heating, this can cause the paint to swell.

Do I need to paint the inner sleeve of the screw?

The inner sleeve that comes into contact with the gearbox shaft does not need to be painted, on the contrary, it must be clean and lubricated with a special lubricant to prevent the fitting, the paint in this place will quickly wear off and can clog lubrication channels or disrupt the fitting of the screw.

Pay special attention to the ends of the blades and the back edge, and these are the areas that experience maximum pressure and are most often in contact with bottom sediments, and here we recommend applying an additional, third layer of paint to enhance protection, and also remember to paint the screw attachment nut, previously lubricating its thread with an anti-corrosion compound.

Comparative table of materials

To make it easier to choose the right composition, we suggest that you look at the comparative specifications of the most common coating types, based on practical experience in different conditions.

Type of coating	Resistance to impact	Anti-corrosion protection	Difficulty of application	Duration of service (seasons)
alkyd enamel	Medium	Good.	Low.	1-2
Two-component polyurethane	High.	Great.	High.	3-5
Epoxy soil with zinc	Medium	Excellent.	Medium	2-4
Aerosol disc paint	Low.	Weak.	Low.	0.5-1

As you can see from the table, saving on material often leads to the need to repaint the screw each season, which ultimately costs more and takes more time. Two-component compositionsDespite the high price and complexity of preparing the mixture, they pay off with a long service life and preservation of appearance.

Common Mistakes and How to Avoid Them

Many boat owners, in an effort to save or simplify the process, make mistakes that negate all efforts. The most common of them is painting without dismantling the screw. Painting the screw "in place" without removing it from the datawoot is almost impossible to quality. You will not be able to process the inside of the hub and risk painting splint holes or lubrication channels.

Another common mistake is ignoring the phase. layingApplying the finish enamel directly to metal (especially aluminum) is a way to quickly detach. The soil creates a chemical bond to the metal and a mechanical bond to the paint. Without this intermediate layer, the coating is only held by surface tension, which quickly breaks down.

It's also a mistake to use graphite greases to paint, and graphite conducts electric current and paired with different metals (aluminum screw, steel nut, bronze washer) can create galvanic vapors that enhance corrosion. Use only dielectric lubricants that are compatible with aluminum.

⚠️ Attention: Never apply paint with a thick layer in the hope that it will improve protection. The thick layer dries unevenly: the film forms on the outside, and the solvent remains liquid inside.

Care for painted screw during operation

Once the screw is painted and installed, it requires proper care. The basic rule is not to leave the motor unnecessarily in water, especially in salt water. Even the most persistent coating is micro-damaged over time, and prolonged contact with the electrolyte will trigger corrosive processes in the chipping sites.

Regularly inspect the surface of the blades, if you find even small chips or scratches, immediately make local repairs, clean the damage to metal, degrease and paint with a small amount of the same paint, which will prevent the spread of corrosion under the main coating.

When stored in the off-season, it is recommended to lubricate the screw with conservation lubricant, which will create an additional barrier to moisture and oxygen. Before the next season, the lubricant will need to be thoroughly washed away with a solvent. Remember that the state of the screw directly affects the efficiency of the engine: a rough, damaged surface creates more resistance to water than a smooth, painted one.

Can I paint the screw with a regular car enamel?

Technically, but it's not recommended. Car enamels are not designed to be constantly immersive and cavitation-intensive. They'll quickly lose their luster, become cloudy and peel off. It's better to use specialized marine compounds.

Should I remove the old paint if it is holding well?

If the old coating is free of swelling, chipping to metal and is tightly held, it can not be removed completely, it is enough to thoroughly clean the surface with fine sandpaper (to create roughness), degrease and apply a new layer of soil and paint on top of the old.

How often do you need to repaint the screw?

The frequency depends on the intensity of operation and aggressiveness of the environment. In fresh water, the quality coating is 2-3 seasons. In salt water or with frequent impacts on the sand, the condition should be checked annually, at the beginning of the season.

Does the thickness of the paint layer affect the operation of the engine?

Yes, but not very much if the layer is uniform, but uneven application can disrupt the screw's balance, causing vibrations at high revs, so it's important to apply the paint in thin, uniform layers.

How to degrease the screw before painting?

It is best to use acetone, white spirititis or special sedatives.