A heat exchanger includes a fluid circuit including a fluid inlet manifold, a fluid outlet manifold, and fluid heat exchange channels fluidly connected between the fluid inlet manifold and the fluid outlet manifold. Each channel has an interior, and walls with an inside surface. A method of repairing the heat exchanger includes pumping an epoxy varnish into the interior of at least one of the heat exchange channels; filling in at least one of a) one or more cracks in the walls of the at least one channel, b) one or more apertures in the walls of the at least one channel, or c) one or more spaces between the at least one channel and another component of the heat exchanger with the epoxy varnish; and heating the at least one channel to a temperature in an approved predetermined epoxy varnish temperature range.