Paint transformers

[jeff]

 I have a couple of transformers with some surface rust on them. I want to paint them. Can I just use any ole spraypaint or is there something better to use on transformers? What do you recommend?

[stratele52]

Hi,

Before doing anything read that ;

Transformer rust , friend or foe ?

http://www.mercurymagnetics.com/pages/mainframe.htm

Transformer Rust -- Friend or Foe?
Question: I heard that rust is bad for my transformers. I’m being told that rust causes an increase in eddy current losses which will heat up and further damage my transformer. Rusty transformers will also hurt my tone, so I must remove the rust or replace the transformers to make my amp operate properly and extend its life.
Answer: Nothing could be further from the truth. While iron possesses the necessary magnetic properties, it is also electrically conductive. Eddy currents are circulating currents in the core induced by a magnetic field emitted by the energized primary winding of the transformer. These electric currents are undesirable. The core is acting much like a shorted secondary winding. They are power-robbing losses that convert your guitar playing (input power) into heat that beats up your transformer. This effect is certainly not helping your tone!
On the other hand, rust (iron oxide) is a nonconductor of electric current. The high resistivity of rust reduces eddy current losses. Since iron is also a conductor of electric current and the goal here is to minimize eddy currents, which is why the construction of the core is made up of a stack of thin plates of iron (laminations) which are insulated from each other by an oxide (rust-like) coating. This confines the eddy currents and prevents them from circulating between the laminations. The higher resistance created results in a reduction of power losses within the core.
What’s this mean in plain English? Back in the early years, manufacturers of laminations intentionally exposed their iron to moist air to encourage rust to naturally form on the surfaces as an insulator! Like a fine patina on an antique bronze the rust on your transformer could be viewed as badge of honor. Assuming that your transformer doesn't have other problems like coil damage from excessive moisture exposure (common with paper bobbins), you can actually expect some improvements in output transformer tone and lower losses in power transformers as rust develops.
If you are still bothered by the cosmetic aspect of rust, never scrape off the rust to the point of exposing bare metal because you will bring back those nasty eddy currents. Instead, brush off the loose particles and re-paint the area with varnish. If you want to send your transformer to Mercury, we can re-dip it in varnish and bake it for a small fee. Transformer evaluations are always no charge providing you cover the shipping costs to and from Mercury.

[HotBluePlates]

Simple question: is the rust on the laminations, or on the end-bells?

[P Batty]

I don't have any scientific answer, but I have sprayed transformers with auto primer (Rust-X, I believe) which supposedly "turns rust into primer", then sprayed them with black enamel. This was surface rust, not deep flaking corrosion. It worked well, and has lasted many years- it works much better on transformers than on cars.

[jeff]

Simple question: is the rust on the laminations, or on the end-bells?

Both

[stratele52]

If I remember  well , few years ago , I read a test from Mercury Magnetic with rusty transformer and they said it sound better.

This test should be find at Mercury M

[HotBluePlates]

Then try what Batty said.

I've had transformers with a little rust on the endbells. I unbolted the endbells (transformer still held together by the varnish on the laminations), sanded with ~100-150 grit, prime and paint. Works great and have pretty painted transformers that look like they should be something special.

[Tiny_Daddy]

I am using Rust-oleum Rust Reformer. It's black, topcoat optional. Just spray it on and forget it.

[xm52]

It is true that rust is a good insulator on transformers. I don't like the look and prefer to stabilize them. I brush off any loose rust from the end of the plates and then apply a coating. In the past, I've been advised both by the folks at Mercury Magnetics as well as Hammond Mfg to use a rustoleum type paint or an oil based urathane. In recent years, I've been using insulating varnish products from MG Chemicals.

I use super corona dope (http://www.mgchemicals.com/products/protective-coatings/insulating/super-corona-dope-4226/), and red insulating varnish (http://www.mgchemicals.com/products/protective-coatings/insulating/red-insulating-varnish-4228/). I prefer the last one because it looks like the orange varnishes that were used on vintage transformers. You don't need a lot, a 2oz bottle will do multiple transformers. They have a high dielectric strength, react well to temperature swings, will stick to steel as well as rust, and the do a great job. Just follow the instructions, work in a well ventilated area, and make sure that the coating has cured before using the amp.