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Alodine vs. SEM rattle can

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dirtybirdy80

I'm New Here
I searched the forum and couldn't find an exact answer for my question. I know corrosion protection is a hot topic and age old debate, but here is my question....

I live out in Santa Rosa, CA (KSTS). In the summer its typically bone dry, but its rainy in the winter and we get coastal fog in the evening and morning, so there is moisture in the air at times.

Therefore, I definitely want some form of protection from corrosion. I'm looking for simplicity, durability, and light weight. My two thoughts were...

1. Cleaning the aluminum with mineral spirits or paint thinner and using SEM self etching rattle can primer.

2. Brushing on Alumiprep 33 followed by brushing on Alodine 1201.

I don't want to do both, just to keep the weight down and the cost down.

Any information is greatly appreciated.
 
Over in Concord KCCR

1. Cleaning the aluminum with mineral spirits or paint thinner and using SEM self etching rattle can primer.
Click to expand...

That was my choice for the same reasons you mentioned, simplicity and also
effectiveness.
My choice for cleaning was acetone, never had an issue with primer.
Large parts would get a thorough cleaning with 409 water solution using a pink scotch brite pad before a final wipedown using acetone. 409 has a slight etching effect on aluminum as well as excellent grease solving qualities.
I used this method for all my interior parts but resorted to an edge and alodine prep for the final exterior paint job.

I noticed in an earlier thread that the poster was applying multiple coats of self edge primer. One coat is all it takes, keep it simple and self etch rattle can SEM will do just that for you.
 
I went with alodine vs spray for small parts. I am lazy.

For small parts or ones that will fit in a 10 gallon rubbermaid storage container, I use alodine rather than straight primer. A primer has several functions, base coat for adhesion of upper layers, active corrosion retarding due to active ingredients, and/or corrosion prevention by barrier. The Mil Spec test use a scratch test. DoD testing showed that alodine alone was equal to just a light coat of chromate primer, a washcoat. So ,I used this method after learning. The time delay between application and installation is reduced and the dimensional changes are nil w/alodine. Also very few rattle cans have zinc or strontium chromate. Neither of which I want to spray indoors anyway.

I don't know why I care anyway this plane will outlast me by a decade, no matter where is is located. Alodine parts just look cool.
 
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How does zinc phosphate primer (available in rattle can) compare to zinc chromate? Is it at least less toxic?
 
Years ago, a friend who lives ~30 miles from the ocean did a test with 2024 tabs treated with pretty much every combination you could dream up, from bare to alodine to all manner of chromate and epoxy coatings. The tabs were wedged into slots in a 2x4, & dipped in a bucket of sea water every few days, & left lying across the top of the bucket the rest of the time. The alodined sample survived as well as the most elaborate treatment.

But know that you can't 'brush on' the acid etch and then 'brush on' the alodine. Yes, you can use a brush, but you still have to clean properly first, treat for the proper time frame in each step, and rinse properly between and after each step.

Charlie
 
rv7charlie said:
Years ago, a friend who lives ~30 miles from the ocean did a test with 2024 tabs treated with pretty much every combination you could dream up, from bare to alodine to all manner of chromate and epoxy coatings. The tabs were wedged into slots in a 2x4, & dipped in a bucket of sea water every few days, & left lying across the top of the bucket the rest of the time. The alodined sample survived as well as the most elaborate treatment.

But know that you can't 'brush on' the acid etch and then 'brush on' the alodine. Yes, you can use a brush, but you still have to clean properly first, treat for the proper time frame in each step, and rinse properly between and after each step.

Charlie
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Yep, also says the European Space Agency. If you want all the test data, chromates, non-chromates alodine only etc. Salt spray cabinets, fence exposure for years, and the like send me your email by email/pm, not pm. I will send a copy of all my primer documentation generated by many tax dollars. Lots of DATA. Read all this and you will be a primer expert. If you don't, you will believe the TV program Scorpion is all real and based on science.
 
dirtybirdy80 said:
Thanks for the response. Think I'm going to keep it simple and light weight and go with Alumiprep, followed by Alodine.
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Keep in mind, though, that while the Alodine treatment is simple, you are not allowed to let any left over Alodine or water contaminated with it, into the sewer system - IMHO impossible in a hobbyist shop.

This alone was reason enough for me to not use Alodine. Getting caught might result in a VERY expensive learning experience...

We prepared all components of the tail feathers with Prekote and then primed them with Stewart EkoPoxy. This is however quite a bit of work - for the tailcone we will probably apply self etching primer, with a 2" wide foam paint roller, to the contact surfaces.
 
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Gisnar said:
How does zinc phosphate primer (available in rattle can) compare to zinc chromate? Is it at least less toxic?
Click to expand...

I've used Moeller zinc chromate primer in rattle cans. Frankly, it's a poor paint, or at least a poor rattle can sprayer, at least compared to SEM. SEM's a very good paint but doesn't have a chemical corrosion preventive, as far as I know. It's just a barrier primer, intended to be top-coated.

Dave
 
If you're going to go to the expense and hassle of alodining, then make sure you follow that up with a good epoxy primer, or at least a zinc chromate out of a rattle can. The 2-part epoxy primers used on military aircraft is some really tough stuff and easy to spray. But, to get get acceptable coverage, you will be adding some weight. I know, because I sprayed a lot of it back in my old Navy days.

That being said, if you haven't read Scott McDaniels' article in "27 Years of the RVator", you should.

I've been using SEM self-etching primer from a rattle can for everything. I use halogen lamps to keep the surface temp up for curing over a 24 hour period before trying to rivet it. After several weeks at typical garage shelf temperatures, its pretty durable. But, you've gotta put it on in 2 to 3 thin coats, and the surface has got to be CLEAN!

I've just bought some Variprime 615S with 616 reducer. I'm going to try Sam Bovington's wipe-on primer technique because I think it will be even easier, and perhaps even save even an ounce or two more over the rattle can.

I looked at Sam's aircraft, and I can tell you the primer looks pretty hardy to me.
 
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I did both Alodine and then painted over it with SEM rattle cans. Nine years in the South Eastern US and not indication of corrosion.

Here is what I wrote on my website about it.

One other thing, to make the spray can paint flow better, I would shake the cans, place the can(s) in front of my small space heater, and shake them again before each use. Then after a pass or two I would dab the nozzle with a paper towel to remove any paint that collected there and keep it from "spitting".

At the end of each work session, I would lightly scrub the parts with a scotch bright pad, clean them with Alumiprep, rinse, alodine them, rinse, and set them out to dry. The next morning I would get up 15 minutes early and go into the basement and spray them with the SEM primer, shower, eat, and leave for work. When I came home, the parts were ready to assemble.

If you find that the primer would come off in big (or small) flakes, the aluminum was probably still damp when you sprayed it.
 
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I can't see why you'd prime internal parts after alodining (see posts #5 & 6).

Also, I'm no expert on paint, but the references I've seen say that if you alodine, you shouldn't use a self etching primer over it. Logic was, the etch step was done by the acid etch step prior to alodine, and the alodine 'converts' the surface. A self etching primer is trying to remove corrosion that was already removed by the etch/alodine process, so at best it's a waste & at worst, it destroys the alodine process. Not saying the paint won't stick, but corrosion protection is no better (possibly worse), and it's extra weight and extra work (Yuck).

Charlie
 
rv7charlie said:
I can't see why you'd prime internal parts after alodining (see posts #5 & 6).

Also, I'm no expert on paint, but the references I've seen say that if you alodine, you shouldn't use a self etching primer over it. Logic was, the etch step was done by the acid etch step prior to alodine, and the alodine 'converts' the surface. A self etching primer is trying to remove corrosion that was already removed by the etch/alodine process, so at best it's a waste & at worst, it destroys the alodine process. Not saying the paint won't stick, but corrosion protection is no better (possibly worse), and it's extra weight and extra work (Yuck).

Charlie
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Alodine is a chemical conversion coating. It is not durable. It is meant to improve adhesion of the primer. This is why you don't leave bare alodine, and why using a self-etching primer over it is redundant.
 
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