Can You Paint Over Rust?

Can You Paint Over Rust?

Yes, you can paint over rust as long as you use a high quality paint or primer that is specifically designed to encapsulate rust.  There are many lower cost consumer grade rust paints on the market that are designed to have you buy them over again in a relatively short period of time.  These should be avoided for projects that need long term performance or are exposed to harsher environments.  Rust converters can also be cheap shorter-term solutions but they just don’t have the durability of higher quality paints/coatings (city/state government agencies never use them for painting bridges or other infrastructure projects).  Preparation is key to long lasting results for any project and generally speaking, the better the preparation, the more successful the application.

Preparation

At the minimum, any loose paint chips, pack rust (heavy rust that can pop off with a hammer and chisel), and mil scale needs to be removed.  This can be done with metal brushes, electric drills with wire wheel attachments, angle grinders (with flap discs/cup brushes work well), needle guns, or basic scraping tools.  Also, any grease, oil or dirt should be removed as it will interfere with the adhesion of any paint/coating.  This can be done with standard degreasers, citrus cleaners, or by power washing.

The Importance of Salt Removal

As salt will accelerate the corrosion process under a new coat of paint, it needs to be removed.  Salt will be present on car/truck frames, any metal exposed to exhaust, fertilizers, places where it is used to melt snow, or in marine areas.   Salt removal can easily be done by adding a chloride bond breaker like Chlor-rid while power washing.  (Salt can’t be removed by high pressure power washing alone or even by sand blasting).   Removing salts prior to application will significantly extend the life cycle of any paint or coating that you use.

What is the Best Paint for Rusted Metal?

There are a lot of options on the market but there are some key specs to look for when selecting a paint or coating for steel (just look on the product data sheet).

  • Salt Fog Test (ASTM B117): This indicates corrosion resistance.  Most good coatings fall within the 2,000-4,000 hour range or better.  Higher numbers are better.  Some coatings like Rust Grip® are rated as high as 15,000 hours and are designed to be applied directly over rust and used in the harshest environments.
  • Adhesion (ASTM B4541): Should range from 600-1000 psi or higher.  More is better here.
  • Direct Impact Resistance (ASTM D2794): The range for this test is often between 100 in/lbs-150 in/lbs.  Higher numbers indicate greater strength and durability.
  • Abrasion Resistance (ASTM D4060): Measured as a weight loss per number of revolutions (usually 1,000) of a weighted wheel (500 or 1000 grams) on the coating.  The lower the weight loss in grams, the higher the abrasion resistance.

Aren’t Most Rust Preventive Coatings Basically the Same?

Like most things in life, you usually get what you pay for.  You can buy cheap “name brand” rust paint that may last 3-4 years (if you’re lucky), or high quality coatings that can last 10-15 years or longer.  On a typical coating job, usually about 10% of the total cost is for the paint and about 90% is for the rest (labor, prep, cleanup, overhead, etc.).  So, for example, on a typical $5,000 job, about $500 pays for the paint and about $4,500 is for everything else.

Most contractors will use the cheapest materials they can find to cut costs and hope that they can do the whole job over for you again once the paint starts failing.  Keep in mind, most contractors usually won’t recommend high quality paints since they don’t want to wait 10-15 years for your next call!

 

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