Standard Transformers which Triad Magnetics offer come primarily in two configurations; open construction and potted. The open construction types are either wound on a bobbin, directly onto a torroid or rod core, or wound on a paper tube with paper sections between each layer of windings. A ferrite core or laminations (usually made of steel) are then introduced and the finished item is dipped in varnish impregnation. Potted transformers follow a similar process path with the additional step of epoxy potting compound being introduced around the transformer.
The materials that make up a transformer are all quite resistant to most commonly used solvents in a cleaning operation. The most vulnerable component is the impregnation varnish used. These varnishes are formulated to be thinned with benzene derivative solvents such as xylene or toluene. For the potted transformers, the benzene derivative solvents are also recommended to be avoided, although the attack will be much slower and therefore probably render no damage for short exposures (room temperature for less than 2 minutes).
If an aqueous clean is used (often consisting of a 10% to 70% solution of Isopropyl Alcohol and water), it is recommended that an oven bake at least 60°C be included for at least 30 minutes after immersion. The reason for this is that the open construction type transformers have the potential for harboring the water within the windings which will cause oxidation over time.
Source - https://www.triadmagnetics.com/faq/