Summary for zinc chromate polyurethane coating Discussion
Expired zinc chromate polyurethane coating is discussed as a legacy aerospace corrosion-prevention finish for magnesium castings, especially on threads and inserts. William Ganoe explains that the original Miller Protective Coating product is no longer available and asks what specification or industry coating would meet the same intent. The replies focus on two practical paths: Brian Terry advises that if the coating is controlled by a specification or TDS, the safest route is to make test batches with the expired catalyst, then verify viscosity, adhesion, gloss, and cure against the required tests; if not under formal control, a small trial may be acceptable. Another responder says the effort of requalification may exceed the value of a new catalyst can and recommends buying fresh material if possible. The technical advice also notes the coating’s main purpose is corrosion control, with zinc chromate acting as a proven wash primer and polyurethane adding toughness and UV resistance.
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 manufacturer, 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
2004
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 zinc 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
[from DLA]
post treatment (like wash primer) works well. Use it with epoxy primers made to MIL-PRF-23377
[⇦ this spec on DLA] H. 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
[⇦ this spec on DLA]
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
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, California
February 12, 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 A. Hi Nek,
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