Weight gains with inside primer on a 4

Supposedly, the hardest decision is deciding what plane type you want to purchase. So am told. I guess they haven’t spent much time in this section yet. If your head hasn’t exploded after reading all it, then congrats, nothing else will do it.

I’ve read from the beginning on all the primer stuff. I’m leaning towards an epoxy primer which many seem to think gives it one of the best seals for the inside, even though I know it's a heavier primer.

When you chose to prime, did you prime all of your inside, alcad sheets and all? And if so, do you have a rough ball park figure of how much weight this added whether it be epoxy primer or other primer? And for those that didn’t prime the alcad sheets but all other metal parts and faying surfaces, do you have a rough estimate on your primer weight gains? I'm plenty inland enough away from sea water, and haven't ruled out just doing the latter, but if the weight gain isn't too bad for doing all of the inside, I'll probably do just that.

In the RV-4 preview plans it describes one example of a paint job which didn’t consist of any primer for the outside, but took other necessary steps for the paint to lie down properly, and also used the minimum amount of paint to still achieve a gloss like finish with Dupont Imron Polyurethane that only added 15 lbs for the entire plane.

If I can get by on 35 lbs or less for the inside and outside of my plane for everything, I think I’ve been worrying too much about it. If the job is done right, about what can I expect on weight gains?

Can of worms but...

I would use a light etch primer (vari prime) only where things overlap. Wing ribs to skin, HS spar to skin. You get the picture.

What Sid said.

wrongway john,
I have wondered about the same thing with the weight issue. I build and paint custom cars on the side, so I am very familiar with epoxy primer but until I started building an airplane never worried about the weight. So being curious how much it would actually add to the airframe I decided to run a test. I cut 2 identical panels from .020 alclad, weighed them on a gram scale to make sure they were the same, then prepped and primed one just as I would if it was a skin for the plane. I used 2 medium coats and let it cure for a couple days. After weighing the primed panel, I found a weight increase of about 7 grams per square foot using the number of coats above. So about 1/4 ounce of weight per square foot...this will add up when you consider how much area there is to cover inside and out. I did not try this with a lighter primer, so if you use something other than epoxy it would be less, but in my opinion this is the best for corrosion protection, more on that in my next post. hope this helps, tom

Here is something you all may find interesting about corrosion on alclad and may help in your decision to prime the inside of the skins or not. I live near the pacific, so salt corrosion does worry me, and since I am getting started I have not gotton to the point where any of my surfaces are closed up yet. I wanted to see how effective the alclad was against corrosion from saltwater, so I made a bath of about 1 gal of water and a cup of sea salt. I put 2 pieces of aluminum in this, one was a bare alclad sheet, the other was one that I had primed with epoxy paint. So far, they have been immersed in the bath for about a week and a half, and I cannot see any corrosion on either one. So for now it appears that the alclad is very effective, but I am going to continue this test until I see something start to fail. I am going to do this again but this time I will add a dissimilar metal to the bath to see if I can get some galvanic corrosion to occur, since I am thinking that may be needed for the test to give any results. I will keep you posted as I find anything new, but right now I am leaning on not primering the alclad interior parts unless the coating is damaged or it is a place where 2 parts touch. I cannot imagine a scenerio in which my airplane will be immersed in seawater for a couple of weeks, and if it is then I don't think my corrosion worries will be the biggest thing I need to be concerned with...! tom

tom gort said:

Here is something you all may find interesting about corrosion on alclad and may help in your decision to prime the inside of the skins or not. I live near the pacific, so salt corrosion does worry me, and since I am getting started I have not gotton to the point where any of my surfaces are closed up yet. I wanted to see how effective the alclad was against corrosion from saltwater, so I made a bath of about 1 gal of water and a cup of sea salt. I put 2 pieces of aluminum in this, one was a bare alclad sheet, the other was one that I had primed with epoxy paint. So far, they have been immersed in the bath for about a week and a half, and I cannot see any corrosion on either one. So for now it appears that the alclad is very effective, but I am going to continue this test until I see something start to fail. I am going to do this again but this time I will add a dissimilar metal to the bath to see if I can get some galvanic corrosion to occur, since I am thinking that may be needed for the test to give any results. I will keep you posted as I find anything new, but right now I am leaning on not primering the alclad interior parts unless the coating is damaged or it is a place where 2 parts touch. I cannot imagine a scenerio in which my airplane will be immersed in seawater for a couple of weeks, and if it is then I don't think my corrosion worries will be the biggest thing I need to be concerned with...! tom

Click to expand...

You also need oxygen to make it corrode. It may not corrode much at all in dead stand still salt water. The usual method is to do salt water spray tests.

Immersion Not a Good Test

Immersion excludes a lot of Oxygen, that's why Salt Spray is the standard.

Hans

that is a very valid point, I didnt think to much about that but it is pretty obvious now that you say it. I suppose it would be hard to get any oxygen when the parts are under water. So I have stopped the test and will try to find some other data out there that has already been found doing the tests in the correct way. thanks for the input, tom

Your number...

tom gort said:

....... So about 1/4 ounce of weight per square foot...this will add up when you consider how much area there is to cover inside and out. I did not try this with a lighter primer, so if you use something other than epoxy it would be less, but in my opinion this is the best for corrosion protection, more on that in my next post. hope this helps, tom

Click to expand...

...is pretty consistent with this data for Sterling U-1201 (a Mil-Spec MIL-P-23377 epoxy primer)

They show 0.213 oz/sq. ft. for a 1 mil thickness

http://www.detcomarine.com/slacguide.htm#DRY WEIGHT PER SQUARE FOOT.

But 1.0 mil is the maximum thickness, recommended is 0.50 mils to 1.0 mil, so perhaps a reasonable weight is nearer 1/8 oz per sq ft if you applied it lighter.

The Mil-Spec tests for an aluminum panel primed for 1000 hrs in a salt spray cabinet.

Interestingly enough, the later versions of the spec (MIL-PRF-23377) require 2000 hrs salt spray - at 0.6 to 0.9 mil dry thickness

http://www.everyspec.com/MIL-PRF/MI...spec=MIL-PRF-23377J__W_AMENDMENT-2.010261.pdf

Add sunshine and wind...

nucleus said:

Immersion excludes a lot of Oxygen, that's why Salt Spray is the standard.

Hans

Click to expand...

....to the mix as well. We 'corrosion X' our ag airplanes semi-annually....cheaper and lighter. Just don't do it before you paint, since it migrates between skin overlaps and will pile headaches and fisheyes on your paint attempts,

Best,

I knew you guys wouldn't let me down

Hey Tom, thanks for the weight gain data though, that really is some good stuff! I had pretty much givin? up on this thread.

Having az gilla also substantiate it with his mil-spec epoxy primer certainly gives me a much better estimate now of what I can expect on weight gain. Roughly a ? of an ounce per s.f. isn?t bad at all, so if I do decide doing the whole inside in epoxy, I think I really have been worrying too much about this.

Some very rough math, and I do mean rough and quick, but simple math:

Wings: About a 10?x4' upper area and lower area X 2 = 80 s.f. for one wing. X 2 for the other wing brings it to 160 s.f.. Possible another 40 s.f. for the ribs which bring that to 200 s.f..

Fuselage, possibly 150 s.f..

Complete tail, maybe 100 s.f..

Total= 450 s.f..

Using ? of an ounce per s.f. figure, and 16 ounces in a lb, we can cover 64 s.f for every lb.

450 s.f. divided by 64=7.03 lbs added by priming with epoxy for the inside.

Now going with az gilla?s slightly lighter mil-spec figures for his primer using the maximum 1 mil thickness:

.213 of an ounce will cover 75.1 s.f.. Divided by 450, we get 5.99 lbs.

But since az gilla says they recommend half that thickness, it only comes out to 3 lbs for the entire inside.

Geeze, is my rough math off, or does this sound about right? This seems way too good to be true. I didn?t spend but a few minutes on this, I?m sure my s.f. estimates could definitely use some revising, but still, I doubt it will be off enough to make that much difference even if we doubled the figure.

I appreciate everybody?s feedback, and will be keeping an eye on this thread if others have more to say and want to correct any of my figures.

Thanks everybody!

wrongway john,
your estemates look pretty good, probably a little on the low side but like you said even if the estemates were doubled then it would only be around 12-15lbs total. not to bad when you think about the peace of mind you will have knowing everything is sealed up. I am planning on doing mine this way, I know it is not required but for the sake of not having to worry about parts I will never be able to inspect again when the plane is done I think it is worth it. Good luck with your project, tom

Another way to confirm weight

I used Akzo Nobel BMS10-11 2 part epoxy. I applied a 1 gallon kit (1 gallon epoxy, 1 gallon hardener). The 1 gallon kit weights about 13.4 lbs.

Spec sheet says Volatile Organic Compounds weight is 5.4 lbs per gallon.
http://www.anac.com/products/DataSheets/10p4-3.pdf

That would make it 10.8 lbs of VOC per 13.4 lbs of epoxy mix, leaving us with less than 3 lbs of solids. Therefore I have added 3 lbs by having primed the aircraft interior.

Anyone care to verify? It does sound too good to be true.