Ask the Mechanic: Why is my car rusty again shortly after a paint job?

Hello Paul, a garage did paint work repairs on my car but within two months, all areas were rusty again. What is the best way to ensure that car paint stays in good condition?

Victor

Hello Victor, rust is a chemical process which all cars are susceptible to if their steel or metallic body panels, components and under carriage are exposed to moisture and air. Oxidation is the ensuing chemical process that will turn the hard metallic parts of your car into brittle iron oxides referred to as rust. One of the critical roles of paint is to protect the metal panels and components from rust.

It is possible that the garage that repaired and painted your car did not properly repair or replace corrosion and rust damaged panels before painting over them. While paint protects against rust damage, it does not repair rust-damaged panels. From your own account, your car must have had rust before the paint job and in two months, the same panels were rusty again after the paint job.

To understand corrosion or rust damage to a car, let me describe the three common kinds of rust and how they ought to be fixed; surface rust usually occurs when your car panels suffer small dents, dings or boda-boda scratches. These scratch off the paint layer and expose the steel panels to the rust driving weather elements. This kind of rust is the easiest to deal with if attended too quickly.

Using sand paper, scratches can be removed or panel beating to remove dents. While the coat of paint on the panel will be removed during repair, a new even layer of protective primer and manufacture approved two coat paint code will be applied to protect and shine the panel while matching it to the paint on the rest of the car. If the little dings and scratches are ignored, they develop into scale rust, which is characterised by small pits, blistering or bubbling.

This stage of car body rust needs more intensive and professional attention with a wire brush to scour the rusty areas and sand paper thereafter. Primer, paint and buffing can follow to finish repair. There are no shortcuts. If you do not stop scale rust on time, it will become penetrating rust. This one practically eats your car metallic panels and components. Penetrating rust forms holes and breaks away pieces of metal which affects the integrity of your car.

Repair of this type of rust involves replacing complete body panels or rust damaged components such as frames, brakes and suspension parts. It is more costly and time consuming repair action requiring high level body rebuilding expertise. A cheap paint job will not fix this. After repair, the usual primer, two layer paint and buffing can follow to complete repair.

WHY DOES MY RAV4 ENGINE KEEP STALLING?

Hello Paul, I bought a locally used 2002 Toyota Rav4 UAF series in December last year. When kept in idle for say three to five minutes, it stops and has to be restarted. This problem has persisted despite buying new pistons, spark plugs, fuel pump, OCV sensor, fuel rail sensor and crankshaft, power steering. What am I missing?

Ritah

Hello Ritah, your 2002 Toyota Rav4’s unstable or erratic idling is usually caused by a faulty or dirty throttle potentiometre, leaking air intake pipe or leaking engine vacuum. The throttle unit regulates the air intake which determines the correct air fuel ratio for efficient combustion and good engine performance.

Diagnosis of the throttle unit can be electrical using a diagnostic tool to assess its performance or mechanical by inspecting the throttle valve for damage or restricted movement due to soot accumulation.

The air intake pipe, which delivers air from the air filter to the throttle should also be inspected to rule out a loose connection or tear.

Delivery of unmetred air can also cause rough idle or engine stall. Another cause of erratic idling can be a vacuum leak from one of the engine vacuum pipes often leading to the throttle or valve cover.

Engine vacuum is critical for maintaining stable running.

If it leaks, the engine will experience unstable idling. Discuss these observations with your mechanic and let me know how it goes.

Send sms: mycar (space) your comments and questions to 6933
Or email them to: [email protected]