K

kritsher

Well Known Member
I don't want to start a new primer war; this post is about quantity, not quality. For the record, I'm using rattle can self etching primer (used Marhyde on the tail, trying Napa 7220 now). I just finished priming the main ribs for my left wing, and I don't know if I'm using too much, too little, or if I'm doing it just right.

Today, it took 2 cans of 7220 to paint 14 main ribs. That seems like a lot to me, and so I'm worried I'm using too much paint. I try to put on an even light coat. Right after I spray the paint, when it's wet, it looks good, like a nice even translucent coat. The part looks completely covered. But, when when the paint dries, it starts to look splotchy (with some parts darker than others) and also "speckly" like there are little pinholes with what looks like bare metal peeking through. That makes me think I need to put on another coat. But I've already used 2 cans!

I want to keep the paint as light and thin as possible, but I want adequate protection.

How do I know I've put on enough, but not too much?
 
Typically if you ask, you are using too much. Very seldom have I seen too little.
Too much just adds weight and cost.
 
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Would you mind posting some pictures of what you are talking about. I am no expert on the subject but I will be facing some of the same concerns you currently are and I wouldn't mind seeing how much you are putting on. Maybe the pics will give someone a better idea of whether or not you are going to heavy.
 
I was told to "dust" the parts with the primer. You're not painting. Just priming. Light dust, and move on. It took me 1 can to do all the ribs for both wings on my -7A.
 
i had the same problem and just sprayed over again. i dont even know how many cans ive gone through but at 10 bucks a piece and just started the fuse, i sure wish it was less cans. some will say its just fine if just sprayed once but if not prepped right it will just scratch or peel off. i like the look of the 7220 gray so i just spray it on till fully covered.
 
Primer coats

I was taught during a Luscombe rebuild in the 1960's, that primer (Zinc Chromate was "THE" primer then) to spray the coats even but only enough that if the surface was a newspaper, the print would still be readable after you are finished. Based upon this lesson I usually thin primer 50% to keep the coat light and the type, brand, etc. of the primer doesn't seem to matter much. I am on aircraft project number 8 now and have used various products but still follow this original rule especially on the interior structure.

Dick DeCramer
RV6 N500DD 300 hrs. flying
RV8 Fuselage
Northfield, MN
 
It depends on what kind of primer you are using and the material.

Alclad do not generaly need any primer because the cladding is anodic to the core and protect the core electrolytically. The only real reason to prime alclad is to make it waterproof and/or protect it electrochemically along rivet lines etc where water can collect and stay for longer periods of time.

Non clad material needs real protection, waterproof and preferably with chromate.

So how thick a layer? It depends. Chromated epoxy will give you a very thin waterproof and tough layer with added electrochemical protection (There are Strontium Chromate epoxy primers made especially for this purpose, and they are sold by most manufacturers of industrial aviation paint). Zinc Chromate spray cans does not need to to give you a waterproof layer, but protects the aluminum electrochemically when water enters, and only a very thin layer is needed. Non chromate stuff will only protect by blocking out water, so the layer has to be as thick as needed to make it waterproof (low pigment wash primers are probably best in this respect with a very thin waterproof layer). Many self etching non chromate primers give a (sometimes thick) porous layer and that does not sound very waterprooof to me.
 
avee8tor13 said:
When I first started I primed everything, lightly then I primed just enough to cover the mating surfaces where two pieces of aluminum came together (rib and wing skin). Check out this link.

http://www.avee8tor.smugmug.com/gallery/2336280_Yo3ae#122236989_6dRtk

You will notice I left the open areas alone, they are alclad coated.

Ignore the old ugly fat guy.
Click to expand...

I did the same on the wing skins... no need to prime everything. Just mating surfaces (in regards to skins...)
 
It just dawned on me. Lots of discussion on weight by adding primer, but what does it really weigh?

Consider that most of the weight is probably solvent, there is overspray, esp on small pieces....what does it really add?

Has anybody weighed the bare metal and then the primed metal and let us know how much weight a can of primer adds?
 
Weight of Primer

Webb said:
It just dawned on me. Lots of discussion on weight by adding primer, but what does it really weigh?

Consider that most of the weight is probably solvent, there is overspray, esp on small pieces....what does it really add?

Has anybody weighed the bare metal and then the primed metal and let us know how much weight a can of primer adds?
Click to expand...

Webb,
I used Sterling's U1201/U1202 Mil Spec Primer/Activator. The manufacturer has the data. The following is from the Sterling Aviation Application Guide. Dry film paint thicknesses can be measured using a micrometer, and probably should be done during your practice work before painting your RV.

Sterling's Best paint system

U-1201 (MIL-23377 primer) 0.0007 inch coating 0.0093 #/sq. ft.

So 100 square feet of primer will add just under 1 pound.

Charlie Kuss
 
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