How to apply Cortec correctly?

I read the Forum and the Cortec user's manual. I dilute Cortec with distilled water 4:1. I clean the surface with acetone, rough a bit with scotchbrite and clean with acetone again. I apply the diluted Cortec with a foam brush. The coverage is not ideal but reasonable I believe. Sometimes I have very few bubbles.

But now Cortec dries and I have a problem. Cortec "shrinks" around holes and away from edges leaving something which looks like uncovered surface. The effect should be visible on the picture attached. It is worse on vertical edges.

Since it is my first plane I do not know is it acceptable or not. 

And I still have a question: how to apply Cortec correctly so it does not leave unprotected regions when drying.

Thank you,


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Thanks for posting about this Alex. I was about to ask similar questions.

So far I have been using self etching rattle can primer on the smaller components ( rudder, elevator etc.). I was planning to use the Cortec when I move on to the wings and fuselage. However, the rattle cans are so convenient.

I have heard dilution with water and denatured alcohol. Spray on and brush on.

I would love to hear the latest and greatest intel.


I suspect you're just seeing a difference in surface tension from the way the metal was cut and drilled when manufactured. The Cortec wash primer (VpCI-373) is probably still there, but it cures more smoothly in those areas, so it isn't as evident. Normally, VpCI-373 peels when there's an adhesion problem. It doesn't really shrink. Or at least, I've never heard of it shrinking. However, it's always good to be sure, so I suggest you ask Cortec;


Please post their answers here for others to see.

On a related note, Cortec also makes VpCI -415, a water-based alkaline cleaner and degreaser that you can use to help reduce corrosion after your plane is built. The Coast Guard saw significant corrosion reduction when they started using it instead of Simple Green to clean their aircraft. No, I'm not affiliated with them in any way.

Some reference stuff if you don't already have it:




Hi Bob.

You are right. Of course cortec does not shrink. I tried to find a simple word. It is an effect of adhesion and surface tension. I wonder are the areas around holes and near edges completely dry (no cortec) or just have a thin layer which possibly dried faster and thinner. I do not have tools to check it. The difference is visible both when the cortec is wet and when it is completely dry.

Thank you,


My guess is it's just smoother in those areas, so it's less visible. I doubt it's thinner. But again, I would ask Cortec. It's always nice to be sure.

I use "Extreme Simple Green" to clean and degrease areas on the plane.  It is specifically made for aviation use and meets a Boeing spec.  It's non-corrosive and no hazardous chemicals.  Great aircraft belly cleaner mixed about 50-50 with water!



BTW, a Cortec tip:  Years ago, an engineer at Zenith told me it was OK to assemble parts even if the Cortec was not completely dry.  HOWEVER, don't do that if it is not a final assembly and/or an assembly that will have to be taken apart in the future.  Once it does dry out, it makes a great glue - you'll deform thin aluminum before the Cortec bond gives way.


I can't tell you how to put it on as I have never used it. I did use some self etching primer on mine. I was wondering if you realized that 6061 aluminum is very corrosion inhibited as is?? You may be over thinking it is what I'm saying. Are you planing on floats in Alaska with salt water????

I did do a plans build that took me way to long and don't want to see other people get overwhelmed and not finish there planes.

Build on.


Thank you everybody for the information and advices. 

After some more talking with different people I decided not to apply corrosion protection while building. I am not planning to fly on floats and I do not live near an ocean.

As a plan B I am thinking about applying something like Corrsion-X two or three years later if really needed. 



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