What You Should Not Use Flex Seal On: The Dos and Don’ts

Flex Seal is a versatile rubberized liquid sealant that has become hugely popular thanks to extensive marketing and infomercials showcasing its ability to quickly patch leaks and bond to a wide range of surfaces. This black liquid formula dries to form a flexible, watertight barrier that can seal water, air and moisture.

With its ease of application and sealing capabilities, it’s no wonder Flex Seal is used by many as an emergency leak repair solution. However, while Flex Seal does have many benefits, it’s important to understand that it’s not suitable for every application. There are several surfaces and materials you should avoid using Flex Seal on.

what should you not use flex seal on

This article will provide a comprehensive guide on what not to use Flex Seal for. We’ll cover the main applications that Flex Seal is not well-suited for and discuss why it’s not recommended and alternative options. By understanding the limitations of this popular sealant, you can avoid problems down the line.

An Overview of Flex Seal and Its Capabilities

Before getting into the don’ts, let’s review what Flex Seal excels at. This liquid rubber comes in a can or bucket and must be brushed or rolled onto a clean, dry surface. Flex Seal adheres to a wide range of materials:

  • Metal including steel, aluminum, copper, cast iron
  • Most plastics and PVC
  • Concrete
  • Wood
  • Tile and masonry
  • Canvas
  • Fiberglass
  • Vinyl
  • Drywall

A key advantage of Flex Seal is its fast drying time. It becomes tack-free in 2-3 hours, though full cure time is 24-48 hours. It forms a flexible, protective coating that conforms to the shape of the surface and is resistant to corrosion and abrasion.

Flex Seal can fill gaps up to 1/4 inch wide and seal out moisture, drafts, and leaks through cracks and holes. It even works underwater, making it handy for marine applications. While single coats typically suffice, thicker build-up and improved durability can be achieved with multiple coats.

Now that we’ve covered what Flex Seal excels at, let’s review some of the main applications you’ll want to avoid using it.

Applications to Avoid Using Flex Seal On

While the infomercials showcase Flex Seal being used in all sorts of creative ways, from repairing cracked fish tanks to sealing the bottom of a rowboat, the reality is that it does have limitations. Here are some of the main uses you should not rely on Flex Seal for:

Foam Materials and Other Porous Surfaces

Flex Seal is not recommended for use on porous materials like foam. The liquid formula can be absorbed into such materials’ tiny holes and pores, preventing proper drying and curing.

Using Flex Seal on foam insulation or cushions can cause warping, shrinking, or even dissolving of the foam. The chemicals in the sealant can react with the foam composition. This also goes for other porous surfaces, like unfinished wood, concrete, and natural fabrics.

High Heat Applications Such as Engine Components

Flex Seal is unsuitable for high heat applications like auto radiators, exhaust pipes, or engine components. The sealant is rated for intermittent temperatures up to 250°F when fully cured. However, it cannot withstand consistent high temperatures above 200°F.

Flex Seal is prone to softening, melting, and losing adhesion at elevated temperatures. Using it to patch radiator leaks or seal exhaust components will fail. It also cannot withstand the high pressures inside engines and transmission parts.

Gas Tanks, Oil Pans, and Fuel System Repairs

For similar reasons, Flex Seal should not be used to patch leaks in gas tanks, oil pans, or anywhere in the vehicle’s fuel system. It cannot withstand prolonged exposure to gasoline, oils, and other harsh automotive chemicals.

Gas tanks also involve vapor pressure, vacuum, and potential chemical reactions that Flex Seal is not designed to handle. The results of using it in a fuel system could be detrimental to the engine. Qualified auto technicians should make proper repairs.

Camper and RV Roofs

Although Flex Seal may seem like an easy solution for sealing leaks in camper roofs and RVs, it is not a long-term fix for these applications. Over time, the sealant is prone to cracking, peeling, and losing adhesion as the roof flexes and moves.

The domed shape of camper roofs allows water to pool if leakage occurs, overwhelming Flex Seal patches. Silicone-based roof sealants are better equipped to handle RV roofs’ expansion, contraction, vibration, and moisture exposure.

Confined Spaces Lacking Ventilation

You should never use Flex Seal in an enclosed space without adequate airflow. As the liquid sealant dries, it releases harmful VOCs (volatile organic compounds) and toxic fumes.

Applying Flex Seal in a confined area like a tank or closed storage container can concentrate these vapors and pose a serious health risk. Always use it in a well-ventilated area. Industrial-grade respirators may be needed for large Flex Seal projects.

Plumbing Repairs and Potable Water Systems

While it may seem like a quick-fix for leaky pipes, Flex Seal should not be used for plumbing repairs. Once cured, the sealant is waterproof but not certified for contact with drinking water.

There is potential for chemicals to leach out of the sealant over time, contaminating potable water. Any plumbing repairs should use fittings, parts, and sealants NSF/ANSI 61 certified for potable water contact.

Tires and Other High-Pressure Vessels

Flex Seal should never be used on tires or anything with high pressure levels. Tires involve rapidly fluctuating, high PSI pressures that the sealant cannot withstand.

At best, Flex Seal will blow out of the puncture hole immediately. At worst, it can cause sudden tire blow out at high speeds. Only specially formulated commercial tire sealants should be used for emergency flats.

Similarly, Flex Seal cannot reliably patch holes in pressurized gas cylinders, compressed air tanks, fire extinguishers, and similar high-pressure vessels. It simply lacks the structural integrity to withstand pressure fluctuations.

Thought-Provoking Discussion Points on Flex Seal

Now that we’ve covered the major don’ts, let’s go over some additional discussion points on the proper usage of Flex Seal and its alternatives:

Is Flex Seal a Temporary or Permanent Fix?

There is debate on whether Flex Seal should be considered a temporary emergency fix or a more permanent solution. In most cases, it is realistically only a short term fix.

Flex Seal may hold up for years if applied properly for certain applications, like sealing a cracked fish tank or stopping basement wall leaks. But it is a temporary patch for things like RV roofs and plumbing repairs until a more robust solution is implemented.

Are There Safer Alternatives?

You may want to consider safer, more eco-friendly alternatives for certain jobs. Cyanoacrylate “super glues” make excellent temporary patches less toxic during curing. Silicone-based sealants are more flexible and heat/fuel resistant.

Roof coatings based on EPDM rubber offer durability without harsh chemical fumes during application. Permanent repairs using appropriate materials should be made for critical systems and infrastructure whenever possible.

What Are the Environmental Impacts?

Some environmental concerns exist about Flex Seal’s VOC content and potential to leach chemicals into soil or waterways. Its runoff can pollute ecosystems, especially if used near storm drains or catch basins.

Proper cleanup of spills, safe disposal practices, and avoiding outdoor applications right before rain can help minimize impacts. Consumers should be aware of these effects, especially for large Flex Seal projects.

Key Takeaways on What Not to Use Flex Seal For

To wrap up, here are some key takeaways on what to avoid using Flex Seal on:

  • Flex Seal does not work on porous materials like foam, unfinished wood, and fabrics. It gets absorbed rather than curing properly.
  • Avoid high heat applications like radiators and engine parts. It cannot take consistent temps over 200°F.
  • Never use on gas tanks, oil pans, or fuel systems due to chemical compatibility issues.
  • It is not a good long-term solution for RV roof repairs due to cracking over time.
  • Don’t use in enclosed spaces lacking ventilation due to toxic fumes given off as it cures.
  • Avoid plumbing repairs, especially on potable water systems, due to contamination risks.
  • It cannot reliably patch punctures or holes in high pressure vessels like tires.
  • For certain applications, consider safer alternatives like silicone sealants, roof coatings, or cyanoacrylate glue.

Always check the manufacturer’s instructions and weigh the risks before applying Flex Seal. While it does have many handy emergency uses, it also has distinct limitations. Avoid using it as a quick-fix on critical systems or anywhere hazardous fumes can build up. Flex Seal can be a versatile addition to any home or garage with responsible use.


Flex Seal is a versatile product suitable for DIY projects and emergency leak repairs. However, it’s important to avoid using it on porous surfaces, high heat situations, plumbing systems, or pressurized components. Instead, consider safer alternatives for certain projects. Flex Seal is easy to apply and bonds tenaciously when used as directed. However, it’s crucial to weigh the risks and follow the manufacturer’s instructions for each unique application. By understanding the capabilities and limitations of Flex Seal, you can avoid wasted time and money on ineffective applications and prevent potential hazards caused by improper usage. Always assess each repair project and choose the best tool for the job, even if it’s not trendy.