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topic 30521

Out of date Zinc Chromate Polyurethane Coating

A discussion started in 2004 but continuing through 2019


Q. I am the Engineering manager of a small aerospace manufacture. We currently have a requirement to paint a magnesium casting with Zinc Chromate Polyurethane Coating Paint to Consist of Miller Protective Coating Inc. Of Bridgeport CT. NO. P-2534 Resin No. E-2477 Catalyst & No. T78A Reducer according to the following proportions 1 part by volume resin & 1 part by volume catalyst then mix approx. 3 parts by volume with 1 part by volume reducer. Apply coating to THD of casting & insert. Air dry 2-3 minutes prior to assembly with seal. After assembly air dry 72 hours min or bake at 180-200 °F for 1 hour min. The problem we have is that the manufacture, Miller Protective Coating Inc. of Bridgeport CT., is out of business. I would like to know what specification or industry coating would meet the intent of this Zinc Chromate Polyurethane Coating.

William Ganoe
Manufacturing/ Process Engineer - Vernon, Connecticut, USA


A. First of all, We are an NADCAP controlled aerospace painting facility and use many products for many customers.

Your problem: The primary intent is "corrosion control" as applied to threads and inserts, which we do here also. Zinc Chromate has proven itself as a very effective "wash primer" for decades now. The phosphoric acid in the mix etches itself into the metal while the zinc reacts with moisture and combats it whenever it is present.

We also use strontium chromate primers that work very well indeed, however some people and organizations do not wish to use it. If it is misused, there are some effects that are not welcome to the environment.

It sounds like the requirement for the Miller coating wanted a corrosion preventative but with more resiliency due to the polyurethane vehicle. Polyurethane is tough and withstands ultraviolet rays very well--better than epoxy. You should not exceed .0005 dry film thickness for corrosion prevention. MIL-C-8514 post treatment (like wash primer) works well. Use it with epoxy primers made to MIL-PRF-23377H. These two together will resist most everything. MIL-PRF-23377H provides for an epoxy primer we use for corrosion prevention--type II class c, high solids--bad stuff, but well worth it! Also check out TT-P-1757 [link is to free spec at Defense Logistics Agency,] Alkyd primer, type II class C--chromate based inhibitors. This would be more to polyurethane than the epoxy would, but believe me...all of the above will do the trick.

Good luck

Bill Workman
- Phoenix, Arizona, USA

Re-qualify expired paint catalyst

February 12, 2019

Q. My catalyst is beyond the expiration but my can has never been opened. Is there a way I can do a test to re-qualify it for a longer period of time?

nek platos
shop employee - los angeles ca

simultaneous February 13, 2019

A. Hi Nek,

All things are possible!!

Is your paint controlled by a specification, or are you working commercially, possibly against teh manufacturer's Technical Datasheet (TDS)?

If you are working to a specification, what I would do is make up some paint, using the life expired catalyst and spray some test pieces as defined by the specification and then test them in accordance with the specification, with a minimum being some form of scratch and tape test for adhesion.

If you are working to a TDS, then I'd still make up the paint with the expired catalyst, but now I'd make sure the paint meets the viscosity as advised on the TDS, then paint some panels (probably anodised aluminum or phosphated steel (depending on what you normally coat) and then carry out a scratch and tape test to ensure the paint still adheres. I would also measure the gloss level if it is supposed to be gloss, semi-gloss or matt.

Brian Terry
Aerospace - Yeovil, Somerset, UK

February 13, 2019

A. Hi Nek,
The hassle of doing it yourself is more than the value of a can of catalyst. Is it produced to a specification? Does that specification allow for requalification? Does it refer to OTHER specifications for testing? You'll have to follow these to the letter.
Beyond whether it is allowable, there's the practical stuff: Do you have the equipment? Do you have someone on staff trained to do that type of testing? Does your shop's QMS have procedures in place for requalification, and are they robust enough to survive an audit?
The way your question is worded suggests that the answers to the second set of questions are probably no.
That being said, if you are not subject to any standards or accreditations, why not simply mix up and spray a test batch and look at viscosity, cure time, texture, and adhesion and make a judgement call?
Frankly I'd buy a fresh can of catalyst and save myself the hassle and potential problems, both paint failure and documentation questions, down the line. There's a reason my Maintenance guys are using $400 gallons of expired MILSPEC epoxy to paint random equipment housings around the shop. No doubt you've got a drill press or something that is due for a snazzy new paint job next time you have some expired base component to go with ;)

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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