No, you should not use Flex Seal on a car radiator. While it might seem like a quick fix for a leak, Flex Seal and similar rubberized sealants are not designed to withstand the extreme conditions inside a cooling system. Using it will likely lead to a complete cooling system failure, resulting in much more expensive repairs.
A car radiator operates under high pressure (typically 13-18 PSI) and high temperature, with coolant temperatures often exceeding 200°F (93°C). The primary function of the coolant is to transfer heat away from the engine. Flex Seal is not a permanent or reliable solution for this environment. The product can break down when submerged in coolant, which contains chemicals like ethylene glycol that can degrade the sealant. Furthermore, flakes of the broken-down sealant can circulate through the entire cooling system, clogging the radiator's thin tubes, the heater core, and even the small passages in the engine block. This obstruction prevents proper coolant flow, leading to rapid engine overheating, which can cause severe damage like a warped cylinder head or a blown head gasket.
For a safe temporary repair on a plastic radiator tank or a hose, a dedicated epoxy putty designed for high-temperature radiator repairs is a better option. However, this is only a short-term fix to get you to a repair shop. The only correct long-term solution for a leaking radiator is to replace it or have the leak professionally repaired by a qualified mechanic. The risk of catastrophic engine damage far outweighs the perceived convenience of a spray-on sealant.
| Factor | Car Radiator Environment | Flex Seal Suitability | Risk |
|---|
| Temperature | Consistently 195°F - 230°F (90°C - 110°C) | Not rated for prolonged submersion in high heat | Breakdown, clogging |
| Pressure | 13-18 PSI (as per radiator cap rating) | Can be compromised under constant pressure | Seal failure, sudden leak |
| Chemical Exposure | Ethylene Glycol-based Coolant | Can degrade rubberized coatings | Contamination, sludge formation |
| System Clogging | Thin tubes and delicate components | Particles can flake off and circulate | Blocked radiator, heater core, engine passages |
| Repair Permanence | Requires a permanent, sealed solution | Temporary at best, highly unreliable | Guaranteed future failure |
