Primer

[Andy Walker]

How toxic is S.E.P., and what type of precautions need to be taken when using a rattle-can S.E.P. (ie: respirator, etc.)?
Kevin

I don't think there are any particular hazards specific to SEP. I used it outdoors with a filter mask. I think a filter and decent ventilation are all you need to be safe.

[rizzz]

How toxic is S.E.P., and what type of precautions need to be taken when using a rattle-can S.E.P. (ie: respirator, etc.)?

Kevin

Definitely not as toxic as zinc chromate or alodine.
A mask is all you need.

I'm priming the inside of the fuselage with a grey SEP called "Killrust etch primer" (http://www.wattyl.com.au/library/TDS/D2 ... Primer.PDF).

It's not as good as the prekote & zinc phosphate combination I'm using on the other parts, but it looks much better:


It comes both in rattle cans or in 1/4l containers, I buy the containers and use my spray gun, much cheaper than the rattle cans

[SvingenB]

6061 is much more corrosion resistance than alclad 2024 by orders of magnitude (literally). 6061 is often used in boat hulls that are used in the ocean and fjords (salt water), and they last for decades untreated. 6061 is also about 5-10 times more corrosion resistance to crevice corrosion than 2024 alclad (ref Corrosion of Aluminum and Aluminum Alloys by J.R. Davis). When Sonex say there is no need to prime the airframe, they are very much correct.

But, there are a few things. Boat hulls are designed not to have crevices (mating surfaces etc), they are welded, and steps are taken to protect against galvanic corrosion. Galvanic corrosion will start whenever dissimilar metals are joined and there is moist, oxygen and some salt. Stainless steel and aluminium is a good and safe combination regarding galvanic corrosion - except in marine environments. Although 6061 has the highest resistance against crevice corrosion of all aluminium alloys, it is not entirely fool proof.

IMO if the airplane (6061 and SS rivets) is placed inland, there is absolutely no need for any corrosion protection. It will last for centuries. In coastal areas it will probably last for decades unprotected (compared with a few years for 2024 alclad), but this is very dependent on the local atmospheric conditions regarding moist and salt.

The main issue with the Sonex aircrafts is SS rivets and aluminium causing galvanic corrosion in marine atmosphere. This is a well known problem for all boat owners. Aluminium is good, SS is good, but the combination is bad, causing the aluminium to corrode due to galvanic corrosion when salt is present. The solution is to protect the joints where SS is in contact with aluminium. A jointing compound is especially made for this, it's called Duralac http://www.llewellyn-ryland.co.uk/duralac.html. That's what I am using. Mating surfaces should ideally be separated by some polymer, typically using an epoxy primer.

So, even though my Onex will experience a marine atmosphere, it will be nowhere near as severe as my boat. I probably don't need any protection whatsoever, but Duralac on the rivets won't hurt and galvanic corrosion between the SS rivets and the aluminium sheets is the number one issue IF corrosion is an issue at all. Deep down I know that crevice corrosion will not be a problem on 6061, but a primer on the mating surfaces won't hurt. I have previously used aviation grade strontium chromate epoxy primer (on all surfaces) with excellent results, but it is a PITA to use and toxic as hell. I'm still experimenting with alternatives, so far a marine vinyl primer or a automotive aluminium primer seems like the best choices, but only on the mating surfaces. The point with the marine vinyl primer is that it makes an absolutely water resistant barrier, which is exactly what I need.

Anyway, my two main points are: 1 IF corrosion is a problem (which it most probably isn't), it most certainly is due to galvanic corrosion between the SS rivets and the aluminium sheets and angles in a marine atmosphere (inland this is not an issue at all). By using Duralac on the rivets, this problem is eliminated altogether. Priming the surfaces will not help on this problem, it can even make it worse because the exposed area of the aluminium becomes concentrated around the rivets and thereby increasing the galvanic potential. 2 Sonex recommendation is to use nothing at all, and that is probably an excellent recommendation.

[jmattson]

So, after reading all of the posts here and articles online it seems as if the self etching primer is unneeded on 6061? I bought my project with the fuselage mostly complete-a large portion of it has been sprayed with either self etching primer or a lightweight epoxy. So all of that work that was done was for nothing? Just added weight and expense? That would really suck... Especially since I already started to spray self etching primer on the wing parts and assemblies...

[rizzz]

So, after reading all of the posts here and articles online it seems as if the self etching primer is unneeded on 6061? I bought my project with the fuselage mostly complete-a large portion of it has been sprayed with either self etching primer or a lightweight epoxy. So all of that work that was done was for nothing? Just added weight and expense? That would really suck... Especially since I already started to spray self etching primer on the wing parts and assemblies...

The topic is heavily debated but here's my take on it,

If nothing else, it will certainly add to the resale value of your Sonex,
The 6061 aluminium Sonex might indeed not need corrosion protection like the 2024 RV's do but even if you're convinced of this fact, if/when you ever decide to sell your plane you will need to convince potential buyers of this as well. I don't think somebody would not buy your plane because it was primed on the inside but I am certain there are many who would not buy it if they see it was not primed.

To give you an example, most other members of my local SAAA group are RV builders and I know when I tell them the Sonex actually does not really need corrosion protection because it's built out of 6061 alumiunium, they look at me with disbelief. A lot of those builders have really strong opinions about priming and how this should be done. Some will even still go as far as stating that the only acceptable method is still both alodining and then priming with zinc chromate. Try selling an un-primed aircraft to someone who has been talking to or one of those guys...

[jmattson]

Thanks Michael, this fact alone does seem to make the priming worth the weight and expense. Next question would be about the type of primer I'm using. It's just a generic self etching primer rattle can I picked up at the auto store. This is what my friend who I bought my project off of told me to buy and use. Should I really be using the more expensive stuff like SEM? Thanks again.

[Mike53]

To each his own .I asked Sonex about it and was told it was totally unnecessary and would only add weight .If you look up 6061t6 properties it states -' 6061 is more easily worked and remains resistant to corrosion even when the surface is abraded',I have had some 6061 laying outside now for over 2 years (cutoffs from elevators and rudder) and they look as good as the day I put them out there.
As to resale it will be easy enough for the prospective buyer to inspect the plane for any tell tale signs of corrosion viv a vis inspection plate removal and on the Onex looking right into the wing sections with the wings folded up.Easy to inspect planes today with the inspection cameras selling for under $ 200 today.

[rizzz]

Thanks Michael, this fact alone does seem to make the priming worth the weight and expense. Next question would be about the type of primer I'm using. It's just a generic self etching primer rattle can I picked up at the auto store. This is what my friend who I bought my project off of told me to buy and use. Should I really be using the more expensive stuff like SEM? Thanks again.

I started off with expensive "aviation grade" zinc phosphate primer (also rattle cans), I've used another "aviation grade" product called EkoPrime as well, now I'm using Wattyl Killrust metal etch primer which can be picked up from any paint store here in Aus.

Out of the 3 I find the local paint store etch primer by far the best, it seems to be the most durable of the 3, it does not scratch as easily as the other two and it is by far the easiest to apply as well, it's also very cheap and easily available. If I were to start again, that's what I would use exclusively.

The zinc phosphate was ok but it only came in rattle cans and is therefore quite expensive.

The Ekoprime is my least favourite of the 3 even though it is supposed to be "aviation grade" whatever that means. The only good thing about it is that t's water based and therefore it is easy to clean the spray gun, but it is a pain to apply (it just goes everywhere and drips easily), it is very fragile and any minor handling of a part will damage the primer and its not cheap either.

[rizzz]

To each his own.

As I said in my previous post, the topic is heavily debated :D

[Bryan Cotton]

Any 40's, 50's, 60's, or 70's era Cessna will be unprimed 2024-T3 alclad, except for a minority of airframes. The C140 I was restoring had some corrosion issues. Considering it is 68 years old that is not too bad. I am not sure that an unprimed airplane would be harder to sell than an overweight one.
When I was building my RV4, I alodined and epoxy primed everything. I only built the tail. Had I finished, it would have taken me an extra year of time but it would have lasted forever instead if just 60 years. On the hummelbird I switched to spray bombs, initially ZnCr and then SEM. I like SEM primer. Used it on the 140 too. On the Waiex - no primer for me. I don't care if it will only last 60 years, I will be 107 then. We live in the Midwest. Maybe on the coast it would be worth it.

I guess you can tell which camp I am in! I say prime if it makes you happy and the auto stuff is fine. If you don't want to prime don't fret.