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Hi guys in my build manual they say to use epoxy chromate Is zinc chromate as good ????? any thoughts ??

Flying Indian wrote:Hi guys in my build manual they say to use epoxy chromate Is zinc chromate as good ????? any thoughts ??

G'day Flyin Injun !!!

Here's and explanation for you I found on a relative site :

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Zinc Chromate Primers, Wash Primers and Epoxy Primers

Q. I am an aircraft structural engineer. Usually we use three different kinds of primers--zinc chromate primer, wash primer and epoxy primer. I want to know the difference between these three kind of primers.

A. Wash primers are water-thin coatings of phosphoric acid in solutions of vinyl butyral resin, alcohol and other ingredients. They are generally applied to give a dry film thickness of 0.3-0.5 mils (8-13 microns). The purpose of wash primers is to passivate steel and galvanized surfaces before applying a full bodied primer. Their functions are to passivate the surface and temporarily provide corrosion resistance, and they are used to provide an adhesive base for the next coating. Sometimes, wash primers are also applied to aluminum and other metals to enhance adhesion of the next coats of paint. When treating aluminum and other difficult-to-paint metals, wash primers are often specially formulated so that there is no excess unreacted acid on the surface after is has been applied.

Zinc chromate is simply a corrosion resistant pigment that is added to certain coatings. In the case of a wash primer, the phosphoric acid actually reacts with the metal, whereas in the case of zinc chromate this is not the case. In dry, low humidity weather the zinc chromate does not do anything other than remain in the coating. On the other hand, when the humidity in the air increases, or on a rainy day moisture for the air penetrates the primer coating and slightly dissolves the zinc chromate. The dissolved zinc chromate solution now does react with the underlying metal surface and forms a passive layer (like a blanket|) that prevents corrosion. As soon as the weather becomes dry again the zinc chromate no longer pays a role ... at least until the next time it rains, when the process is repeated. If there are frequent cycles of high and low humidly the zinc chromate will eventually be depleted and soon after you might start to see corrosion of the metal..

Zinc chromate pigments can be added to primers made of several different resin types, such as epoxy, polyurethane, alkyd and others.

On the other hand, there are epoxy, polyurethane, alkyd and other primers that do not contain zinc chromate, but might instead contain other corrosion inhibiting pigments.

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SO ...... by this article ... ( and watching others who have created big "baths" to put allllll their individual aircraft parts IN befoe they rivet them together ) ....
the zinc "wash" would seem to be THE best way to have a complete coating on any parts .......

BUT ..... You have to remember that, yes ..... whilst MOST aircraft are made from 2024-T3 ... which IS "stronger" than 6061-T6 ( what the Rebel is made from ) - it also has CRAPPY corrosion resistance as compared TO 6061-T6 - so it NEEDS to BE treated with anti-corrosion stuff ....... whereas the Rebel largely DOESN'T !

IE : There is no "real need" to treat 6061-T6 ANYWAY !!!

The only disclaimer here is that - where there COULD be problems ... is where skins overlap and create places to retain moisture - and where rivet holes and rivets ARE - as these can also "trap moisture" and because of the dissimilar metals used - these areas have a very much higher chance of setting up electrolysis between the skins and the rivets

The fix here - is to mix up a small batch of 2 pack epoxy zinc chromate - then dip each rivet body IN it and then roll two rivet bodies around each other - which then spreads the epoxy mix evenly over the rivets - then immediately insert and "pull" the rivets .....
The excess epoxy will squeeze out - leaving no air gaps to retain moisture ( so no electrolysis can occur ) ....

I then went along each fresh line of pulled rivets with a rag & thinners or MEK and wiped all the excess epoxy chromate - away

We also then used a plastic syringe ( with no needle in it ) - filled with 2 pack epoxy Araldite / similar slow drying epoxy glue - and filled EVERY rivet hole on the outside - and lightly sanded off the excess glue - to create the look of a round head SOLID rivet
The finished result is that the plane looks like it has been solid riveted - plus - moisture can NEVER get in and sit inside the rivet to rust and create stain lines down the plane ..... a win / win situation ! :thumbup:


I used "aircraft grade" aerosol cans of zinc oxide on our first Rebel ....... between each layer of the skins where they overlapped ONLY ( just to retard ANY galvanic action that might occur when they get water between them )

Remember - Zinc Chromate is apparently HIGHLY toxic ( DON'T breath in any airborn mist etc !!! )

Whereas Zinc Oxide is ( apparently ) relatively harmLESS ........

Hope this helps !!!

Rick in Orrrstraylya ............... and his Injun ! :thumbup:


( the cheese n' kisses ( Misses / Mrs ) and I are both members of a touring / cruising motorbike club down here ...... and we have a good friend who IS a genuine Blackfoot Indian - and HE rides an Indian Scout ( 2015 model ) ....... so he's known as : "Blackfoot Joe" - the Injun ON an Injun .... :thumbup: )

I found THIS pic' recently - and he LOVED it !!! .... it's circa 1918 - and shows an Indian Chief ON an Indian Chief !!!
( notice the sign reads "Motocycles" ........ not "MotoRcycles" ...... )

AFAIK epoxy primer is the only primer that will retain adhesion on our flaperons when polyfiber fabric is glued on.
It also works better than anything else I've tried on my steel engine mount.
Seems to be stupid expensive lately though.
Ken

BMS 10-11 epoxy is the way to go. Paint on the faying surfaces, assemble wet, and dip each rivet, and then hope you never need to take them apart. Just don't get it on the outside surfaces cos paint of any sort won't ever stick or cover it.
Rebel 804 down under.

HI guys can anyone tell me what i should be using before i put this wing together. A canadian source and part number would be great. I bought zinc chromate from aircraft spruce ???????

Hi Flying Indian,
I am like you putting my first wing together and had the same Question as mine is an old kit 540R and had some surface corrosion already.
After consulting with my old colleagues in the rebuild department Flight Line Dunedin I was advised to use DUPONT Corlar 13550S epoxy primer .
This stuff is great ! Sprays easily ,goes along way , and the beauty is it can be used directly under a top coat. Apparently Cessna use this in the factory now.
Down side is it is a two pot so a bit fiddly to mix etc but don't let that put you off as it is well worth it.
I have been washing down parts with clean water and a bit of sugar soap (,will get gib for this but it works ) then scotch bright surface before applying alidine 1201
washing off with clean water allowing to dry then painting.
The Dupont Corlar is a bit expensive but to my mind well worth it.I am going to paint everything bar the fuel thanks as I have spent to long in the past trying to patch up corrosion.
All the best with the build .
Regards
Tim Graham Rebel 540R.

Hello Flying Indian

I'll second what Tim has said, from the beginning we have been using the chromate free version of the Corlar etching primer (810R, now apparently called 13580S). We spoke directly with one of the Dupont tech reps, telling him what we were doing and it was his recommendation. We buy it at our local (Canadian) NAPA auto parts jobber.

As an aside, we also talked with Cessna and have chosen to follow their practice of applying the corrosion inhibiting primer to the piece parts and assembling them dry. I'm sure that some people will disagree with this approach but if it's SOP for Cessna it's good enough for me and it keeps the VOC's out of our assembly shop.

We do not use chromate or Alodyne because of the issues of disposal of the waste material.

Cheers,

Dave
Elite 583

Thanx guys you have been very helpfull.I am a tube and fabric guy and this is my first aluminum project. I lke to get as much info as i can before proceeding. I assume I can mix the epoxy primer and apply with brush on mating parts ??? wet or dry ????? Mine is an older kit 414 I plan on putting inspection holes in wings how important is the extra hole and doubler behind thestrut attach bracket ??? Do the factory wing tips work well or should they be substituded???? I am looking for performance not beauty !!!!!!

Thanx guys you have been very helpfull.I am a tube and fabric guy and this is my first aluminum project. I lke to get as much info as i can before proceeding. I assume I can mix the epoxy primer and apply with brush on mating parts ??? wet or dry ????? Mine is an older kit 414 I plan on putting inspection holes in wings how important is the extra hole and doubler behind thestrut attach bracket ??? Do the factory wing tips work well or should they be substituded???? I am looking for performance not beauty !!!!!!

Hi again guys I gave my naPA dealer the dupont number and he sent me up nason dtm epoxy primer/sealer (grey) a 2 part system. Any thoughts on this stuff??? The sheet says it s for any type of metal including aluminum.Thanx again

My take on it is that any two component epoxy is likely to be better than any single component primer. I used a two component during assemby but I question whether a two component is worth the extra trouble for assembling faying surfaces compared to a single component product.

For exterior surfaces:
Verbally the polyfiber folks I've spoken to strongly recommend alodining for a lasting paint job on exterior surfaces.

Users of other products who don't alodine seem to recommend an etching primer under the epoxy primer on exterior surfaces.

As someone who refuses to use alodine, I used metal prep etc. with epoxy primer on the exterior but after 11 years I am now seeing filiform corrosion under the primer in a few places. Perhaps I did not put on a thick enough coat of epoxy primer in those places. Fortunately it is only on small pieces like struts and fairings. Anyway I'm now setting up to use an etching primer under the epoxy primer for pieces that are getting repainted. To my surprise, price and availability gives the nod to the PTI product from Aircraft Spruce for me. Unfortunately it seems that the etching primers need to be sprayed to get the required very thin coat which probably means they are not suitable for small areas that are touched up with a brush.

I still have not found any evidence of corrosion on interior areas that got a misting of corrosion-x and nothing else. I'm not near salt water but I am in a area that gets frequent condensation.

Ken
amateur painter

Flying Indian wrote:Hi again guys I gave my naPA dealer the dupont number and he sent me up nason dtm epoxy primer/sealer (grey) a 2 part system. Any thoughts on this stuff??? The sheet says it s for any type of metal including aluminum.Thanx again

What is the part number of the material they sent?

Cheers,

Dave
Elite 583

Hello Indian,

The wing inspection hole s let you check both sides of the strut connection at the main spar. I doubt anything bad would ever happen there; I have checked it several times and nothing. It is nice, though to be able to inspect the interior of the wing. I guess sometimes, too, I just like looking at what I built! I'd build them in! Craig

Hi guys The part number is 491-10 made by nason

HI AGAIN guys I AM THINKING OF USING THE ZINC CHROMATE TO ASSEMBLE MY PIECES TOGETHER THEN SPRAYING THE SURFACES WITH EPOXY PRIMER ANY THOUGHTS ON THIS????? iS ANYONE USING ZINC CHROMATE ANY MORE ????