Best Epoxy Putty for Cast Iron (2026)

A cast iron exhaust manifold comes off the truck with a hairline crack along the flange, and a replacement means waiting on parts and pulling the whole job apart. A stick of putty that molds by hand, bonds to cast iron, and hardens into a patch that can be sanded and drilled sounds like the right shortcut, and that is where the best epoxy putty for cast iron earns its place in the garage. This roundup compares the sticks on set time, heat resistance, strength and finish, so the pick matches the job.

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Picks for the garage

#	Product	Shop Score	Buy
1	J-B Weld 8297 HighHeat 500 Degree Epoxy Putty	9.7/10	Buy on Amazon
2	Flex Seal Epoxy Ultimate Putty Waterproof	9.6/10	Buy on Amazon
3	Steel Reinforced Epoxy Putty	9.5/10	Buy on Amazon
4	Steel Reinforced Epoxy Putty	9.4/10	Buy on Amazon

1

J-B Weld 8297 HighHeat 500 Degree Epoxy Putty

J-B Weld·9.7/10 Shop Score

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The Shop Score runs from 9.2 to 9.8 and weighs performance (35%), build (25%), value (25%), safety (10%, where it applies) and warranty (5%), using only what the manufacturer and the listing publish. How we pick ›

A hand-mixable two-part epoxy putty stick made for high-temperature repair, with 800 PSI lap shear strength and a 1 hour set time, listed for engine blocks, exhaust manifolds, tail pipes and mufflers.

Check price on Amazon

Pros

• 800 PSI lap shear strength at room temperature, 600 PSI at 400°F
• Withstands 450°F continuous heat and 500°F intermittent heat
• Listed for engine blocks, exhaust manifolds, tail pipes and mufflers
• Can be tapped and drilled

Cons

• Sets in 1 hour, with 8 hours to full hardness, so the job waits
• Manufacturer warranty is listed with no term stated
• One 2 oz stick in the box
• Grey only, with no color choice stated

2

Flex Seal Epoxy Ultimate Putty Waterproof

Flex Seal·9.6/10 Shop Score

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The Shop Score runs from 9.2 to 9.8 and weighs performance (35%), build (25%), value (25%), safety (10%, where it applies) and warranty (5%), using only what the manufacturer and the listing publish. How we pick ›

A moldable 2 oz epoxy putty stick that sets in 15 min, holds 4 tons, and forms a waterproof barrier once fully set, for metal, wood, plastic, ceramic, concrete, pipes and marine use.

Check price on Amazon

Pros

• Sets in 15 min with no drip
• Holds 4 tons from one 2 oz stick
• Withstands up to 350°F once fully set
• Impact, chemical and fuel resistant, waterproof once set

Cons

• No warranty term is stated
• No lap shear strength figure is stated
• Dark Grey only
• One 2 oz stick in the box

3

Steel Reinforced Epoxy Putty

Wtnukt·9.5/10 Shop Score

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The Shop Score runs from 9.2 to 9.8 and weighs performance (35%), build (25%), value (25%), safety (10%, where it applies) and warranty (5%), using only what the manufacturer and the listing publish. How we pick ›

A steel-reinforced 2-pack AB epoxy putty kit that fills cracks, holes, dents, chips and uneven surfaces on metal, wood, ceramic, stone, tile, plastic, glass and concrete.

Check price on Amazon

Pros

• Two sticks in the kit
• Steel-blended formula for cracks, holes, dents and chips
• Bonds metal, wood, ceramic, stone, tile, plastic, glass and concrete
• Waterproof, heat-resistant and mild chemical-resistant

Cons

• No set time is stated
• No heat figure is stated
• No strength figure is stated
• No warranty term is stated

4

Steel Reinforced Epoxy Putty

Generic·9.4/10 Shop Score

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The Shop Score runs from 9.2 to 9.8 and weighs performance (35%), build (25%), value (25%), safety (10%, where it applies) and warranty (5%), using only what the manufacturer and the listing publish. How we pick ›

An all-purpose steel-reinforced epoxy putty kit that mixes equal parts A and B, fills gaps, cracks, fissures and dents, and bonds metal, wood, ceramic, stone, tile and plastic.

Check price on Amazon

Pros

• Equal-parts A and B mix until the color is even
• Fills gaps, cracks, fissures, dents and surface damage
• Bonds metal, wood, ceramic, stone, tile and plastic
• Sands, drills, shapes and paints after hardening

Cons

• No set time is stated
• No heat figure is stated
• No strength figure is stated
• No warranty term is stated

Compared on paper

Set times, heat figures and strength figures are manufacturer stated, as published on each listing and collected 1 November 2026. Where a listing states no figure, the table says so rather than guessing.

How the scoring works

Each stick is scored on performance, build, value, safety and warranty, with the weights published on the How We Pick page. Safety points need a stated safety listing or a safety-relevant feature, and warranty points need a stated warranty term, so a listing that states neither scores zero in both.

Shop Scores range from a low of 9.2 to a high of 9.8, with each tool positioned by its weighted points proportionally, and more points always yields a higher score. A tool with no power source and no listed safety feature has safety removed, and the score is taken over 90 points.

What the listings tell you

The four listings in this roundup all describe hand-mixable two-part putty sticks, and all four name metal among the surfaces they bond. Only the J-B Weld listing states a lap shear strength figure, and only two listings state a set time, so most of the comparison rests on heat figures, stated uses and finish. The J-B Weld listing also states a manufacturer warranty, but with no term, so no pick here earns warranty points.

Buying guide: best epoxy putty for cast iron

Match the heat to the job

Cast iron on an engine block or exhaust manifold runs hot, so the heat figure matters more than anything else here. The J-B Weld listing states 450°F continuous heat and 500°F intermittent heat, while the Flex Seal listing states up to 350°F once fully set. The other two listings call their putty heat-resistant with no figure stated, so a hot job should go to a stick with a stated figure.

Set time decides the pace

The Flex Seal stick sets in 15 min, which suits a quick patch that needs shaping fast, while the J-B Weld stick takes 1 hour to set and 8 hours to full hardness, which suits a repair that can sit overnight. The other two listings state no set time, so the pace of the job is a guess with those.

Strength figures, or the lack of them

Only the J-B Weld listing states a strength figure, with 800 PSI lap shear strength at room temperature and 600 PSI at 400°F, and the Flex Seal listing states that one 2 oz stick lifts and holds 4 tons. The other two listings state no strength figure, so their holding power rests on the maker’s word rather than a number.

Finish matters on a visible repair

All four listings state that the hardened patch can be sanded, drilled and painted, and the J-B Weld and Wtnukt listings add tapping and threading. A repair on a visible cast iron part should be shaped while soft and sanded smooth after hardening, then painted to match the surface.

Check the pack before the job

The Wtnukt listing describes a 2-pack kit, which leaves a spare stick for the next crack, while the other listings describe a single stick. The J-B Weld and Flex Seal listings both state a 2 oz stick, and the Wtnukt listing states a weight of 0.1 lb. Check the amount on hand against the size of the crack before mixing.

Putty and filler are not the same repair, so the epoxy putty vs body filler comparison sorts out which one suits a cast iron crack, and the best epoxy putty for aluminum roundup covers the lighter metal in the shop.

Garage Fit Check

• Space: the Flex Seal package measures 6.0 x 4.0 x 0.75 in, so a stick fits in a toolbox drawer and the repair needs only a bench and a steady hand.
• Power: none of the four listings states any power need, since each stick is mixed and shaped by hand.
• Storage: the Wtnukt listing states to seal leftover material tightly for later reuse, so an opened stick stays usable for the next repair.
• Before starting the job: check that metal is among the surfaces the listing names, and match the stated heat figure to the part.

FAQ: garage tools

What is an epoxy putty?

An epoxy putty is a hand-mixable two-part stick that bonds, fills and patches hard surfaces. The listings here describe sticks that fill cracks, holes, dents and gaps on metal, wood, ceramic, stone, tile, plastic, glass and concrete, and harden into a layer that can be sanded, drilled and painted.

Does epoxy putty stick to plastic?

Yes, according to the listings here: the Flex Seal, Wtnukt and generic listings all name plastic among the surfaces they bond, and the Flex Seal listing also names pipes and marine use. The J-B Weld listing names high-temperature automotive parts rather than plastic, so a plastic repair should go to one of the three that names it.

How much is epoxy putty?

The listings in this roundup state no price, so check the listing for the current price before buying. The size stated is a 2 oz stick for the J-B Weld and Flex Seal picks, and a 2-pack kit for the Wtnukt pick.

How do you remove epoxy putty from metal?

The listings describe a hardened patch as sandable and drillable, but state no removal method, so follow the guidance on the listing or the maker for the stick in hand.

How do you remove epoxy putty from a pipe?

The listings here state no removal method for pipe either. The Flex Seal listing names pipes among its uses and states the patch can be sanded, drilled, tapped and painted, but removal itself is not covered, so follow the guidance on the listing or the maker.

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