Types of Primers

Hi Everyone,

I'm a newbie and started the daunting task of doing research on primers, and I have to say I'm now even more confused then when I started. (etching, non etching, requires topcoat vs doesn't, epoxy based vs water based, primers vs sealers, conversion coatings etc...etc... you see what I mean )

I thought it would be nice if someone in the know could explain the different types of primers available for aircraft and a short discription on it's application method and use.

This could be a nice reference for future newbies.

Looks like there are a couple people in the know on this forum, judging from the some of the primer topics. [highflight, f1rocket, ptrotter or any other takers?]

Maybe there is a website reference already explaining this, please post it here then.

Thank you in advance.

Vern said:

The only thing I'd like to clarify is that ALL primers, no matter what their chemical make up, are intended to be used under a topcoat of some sort. That's what it means for a product to be called a "primer".
That goes for epoxy primers, of course, but the one significant difference is that epoxy primers have only a narrow "open" time during which the crosslink chemical process has not yet, well, linked (obviously, that's why there's a catalyst involved).

Until that happens, epoxy primer DOES retain it's porosity to encourage better paint adhesion, but there's even more to it than that.
In any paint product, you have two kinds of adhesion properties to deal with; chemical adhesion where the chemistry of the topcoat actually reacts and bonds chemically with the primer (that's the best kind), and then mechanical adhesion where the topcoat "grips" the properly cleaned and sanded primer.

While epoxy is still in it's uncured open stage, it remains both chemically receptive to whatever chemically compatable finish is put on top of it, and it also remains porous where it encourages mechanical adhesion to the paint as well.
After epoxy primer cures, what happens is that it becomes a really good barrier against most anything (almost as good a barrier as paint itself), including any paint you might want to put on top of it.

So what you get when an epoxy cures beyond it's open stage is an epoxy primer that becomes worthless for it's originally intended purpose as a substrate, but great as a protective barrier as long as it's not left to weather in the elements as an external coating.
Beyond this curing time (note the term "cure", not "dry"), it will no longer chemically bond with any topcoat and, to reiterate, the result is that the cured epoxy primer then becomes a very good "barrier" and you would find that any paint put on top of it would be prone to chipping and peeling even if you sanded it well (sanding would only improve mechanical adhesion, and that's not good enough for a long term, durable finish).

That's why if you let PPG DP/LF (as an example) sit for longer than a week and you THEN want to top coat it, you don't have to take it off, but you DO have to sand it thoroughly and then reseal it with another coat of itself before painting it.

The short story, as if I could ever tell a short story, is that there's more to the story, and that's why the discussion will never end.

Vern
RV7-A

Click to expand...

I lifted this from another thread. Vern does a good job of explaining it.

Thanx Randy,
That is a very good start to this thread.
Maybe some others can share some more information of the different types and application methods?

Yer killin' yerself, Rudy, yer killin' yerself.

The reason why I don't jump in all the time to answer primer/paint questions is because the subject is so "large" and the more we talk about it, the more confusing it gets.
Each specific question begs a specific answer because of the various chemical "intents" of each of the products, and things get more confusing when you consider that no one product is designed to do something without another product within the brand or system.

You started this thread with way too general of a question, so I'll throw in some general ideas to help get you in the right direction, and keep in mind that I am talking ONLY about RV's, what they are made of, and what we can do to protect them or make them pretty.

A. The inside surfaces of our RV's are protected from corrosion by being coated with pure aluminum. That's what "alclad" means. This pure aluminum is more resistant to corrosion than is the aluminum alloy it is applied to.
So, generally, Van's statement that you do NOT have to add any further protection to the inside of his kits is correct. However, there ARE reasons why we probably should go further and do some additional corrosion protection, and that's because we strip away areas of alclad when we machine prep the panels for riveting, and we also (probably) scratch parts of panels as well. So for a "perfect" aircraft, most of us do some kind of additional corrosion protection if nothing more than using an Alodine "pen", or maybe just priming the edges where all the riveting is done.

B. Once you decide to add additional corrosion protection to the inside panels, you are then faced with way too much information about chemistry, methods, brands, procedures, etc. This confusion leads some people to search for the "perfect primer" that is absolutely better than everything else. After all, since there seems to be lot's of choices, then it stands to reason that there MUST be a "best" choice, right? (Hint: There is no "best" primer or system, but there are several good ways to go.)

C. There are three subsets of base metal protection. They are:
1. "Conversion" chemicals that react with the aluminum chemically. Examples of these would be the ubiquitous Alodine, or the Sanchem products and others. These are the "messy" products that require preprep chemical cleaning and thorough rinsing. If you enjoy giving your dog a bath, then this system is for you.
2. Primers that react with the aluminum chemically. An example of this type would be Veriprime and Zinc-Chromate, but there are others.
3. Primers that don't chemically react with the aluminum, but sprayed on properly CLEANED aluminum, add long term protection to (inside surfaces) almost as good as using a complete paint system. As a group, these would be the epoxy based primers
A couple of additional points here are that, even though the above 3 types of products add protection to aluminum, they are ALL designed to be used UNDER a topcoat, and that goes especially for the conversion chemicals like Alodine. Left uncoated, conversion chemicals offer at best, only the same protection as alclad which is why some guys just use an Alodine pen to cover scratches rather than do the whole panels.
Primers in the Zinc-Chromate and Veriprime class (Zinc-Chromate has been replaced by safer and better products like Veriprime) are also designed to be topcoated, but do offer a very minimal amount of a moisture barrier (but not much).
Spraying a primer of the type mentioned in item #2 on top of a conversion coating described in item #1 is a waste of time and may even cause a LOSS of adhesion over time because #2 primers REPLACE conversion coatings as a "process". These are the "self etching" class of primers.
Another point is that enamel based and lacquer based primers are totally worthless for the job.

D. Don't get hung up on brands because any of the well known brands offer excellent products of the types noted in the preceding paragraph. HOWEVER, once you choose a brand, stay with it. Chemical formulations are generally similar for similar products, but manufactures spend lots of big $$$ in research to make sure that each of their products work like they are supposed to with their other products. Unless you consider yourself a "pro" and have knowledge of cross brand compatibility, stay with the same brand all the way to the final paint finish.
Over the years, I have used most of them including Dupont, Sherwin Williams, PPG (Ditzler), and a couple of foreign brands. In the end, I settled on PPG as my only "system" so whenever you ask me a specific question, I will usually phrase an answer around PPG products. However, you won't go wrong with any of the major brands as long as you stay WITH that brand to the end.

E. For me to tell you the "best" way to prep/prime the inside surfaces of your RV is like telling you that the guy I voted for in the last election is the "best" president for our country. He is (Bush), but lot's of people will disagree and tell you otherwise. That's the way it is when discussing primers.
What I offer here is simply MY thoughts on a good way to go with inside priming so here we go:
1. If you don't prime inside at all, your aircraft will still be flying years after your next of kin inherits your RV.
2. If you do want to prime, don't make it a big whoopty-do; make it as easy as possible on yourself, yet do it in a way that will give you REAL benefit and not just a good "feeling" about it.
3. Remember that we are working with brand new alclad panels. They are not recycled, or used, or have any corrosion problem that has to be "fixed". Because of that, and in my opinion, that means that we don't really HAVE to get down and dirty on pre-prep. That means that if you want the easy way out, you can go with a self etching primer such as Veriprime and not have to use a conversion process. As I mentioned earlier, you do NOT want to use a conversion process UNDER a self etching primer such as Veriprime. All you need to do is to properly clean (CLEAN)... (did I mention; clean?) the bare aluminum before spraying.
4. If you will be hangaring your aircraft, then Veriprime only is an easy, and effective, way to go. However, Veriprime isn't as good as a moisture barrier as is epoxy primer (such as the PPG DP/LF), so if you think your RV will be spending most of it's life outside, then I might consider using a conversion coating (again, Sanchem 6100 seems to be a bit less messy than Alodine), but then spray a coat of epoxy primer over it. Once cured, the epoxy primers are a better moisture barrier (but still not as good as if it were top coated with urethane paint... but don't go there).
I'll even say that, because our parts are brand new, and if you want to use epoxy primer, you don't really need to use a conversion process first. Spraying the epoxy on top of brand new and properly cleaned (no wax or grease residue) alclad will be way more than enough long-term protection.

Now, let confusion reign.

Highflight

Fantastic write up Vern, that was exactly what I was looking for, thanx a lot for the effort.

Now all we need is that one off the forum moderators to take your comments and make them into a sticky note: "Introduction to primers" this way it will always be on top of the primers forum and easy to find by new guys.

Another consideration BEFORE you Alodine

I want to thank you Vern for an excellent and informative article. I also wanted to add the following that I found about the Alodine process. This is under the technical information at: http://www.alphametal.com/index.html
"Anodizing Aluminum assemblies
Aluminum parts that have been assembled together will anodize successfully. However the designer should be aware that lap joints such as spot welds and joints that are not completely watertight can become traps for cleaning agents and sulfuric acid. In some cases it is impossible to completely rinse all of the sulfuric acid from the joint. If this happens the joint then becomes a site of increased corrosion potential............. . However, as above, it is much better practice to consider this potential problem during the design phase of your product and plan to do the anodizing before assembly if at all possible. "

This information and Vern's information about the self-etching primers has saved me a lot of work. I found the above information as the result of a search that I did when I was wondering about the acid that almost for sure is held in by capillary action around the seams and rivets when doing the acid wash during the alodine process.