Possible priming alternative.

[Mel]

Just a word here on priming.....You guys are going WAY overboard on scuffing prior to priming. You don't need sandpaper, especially 120 grit. Just use red Scotchbrite. All you need to do is break the glaze. This will NOT remove the alclad. 2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!

[lr172]

Originally Posted by Mel

Just a word here on priming.....You guys are going WAY overboard on scuffing prior to priming. You don't need sandpaper, especially 120 grit. Just use red Scotchbrite. All you need to do is break the glaze. This will NOT remove the alclad. 2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!

I agressively abraded the exterior during paint to get a better mechanical adhesion and my primer stuck like glue. This was based upon the recommendation from a paint manufacturer. the more agressive the grit, the better the bond, assuming the paint can still leave a smooth finish. It should be noted that I did this with a DA sander and 180 on a DA leaves a finish similar to 220.

I wasn't recommending this approach for others, only passing on a relevant data point about alclad thickness.

Larry

[rocketman1988]

To say that 2024 aluminum is "VERY corrosive" may be a bit of an embellishment...

Is the 2XXX series more prone to corrosion than other alloys? Yes, but you have to put that in perspective.


Here is a reference concerning corrosion of aluminum...probably more than you ever wanted to know...

https://www.arl.army.mil/arlreports/...RL-TR-4937.pdf

There are numerous other references to cite, as well...Here is an excerpt from an ASM International text titled "Corrosion of Aluminum and Aluminum Alloys":

"Corrosion Resistance. When aluminum surfaces are exposed to the atmosphere, a thin invisible oxide skin forms immediately, which protects the metal from further oxidation. This set protecting characteristic gives aluminum its high resistance to corrosion. Unless exposed to some substance or condition that destroys this protective oxide coating, the metal remains fully protected against corrosion. Aluminum's highly resistant to weathering, even in industrial atmospheres that often corrode other metals..."

and...

"Aerospace. Aluminum is used in virtually all segments of the aircraft, missile, and spacecraft industry--in airframes, engines, accessories, and tankage for liquid fuel and oxidizers. Aluminum is is widely used because of its high strength to density ratio, corrosion resistance, an weight efficiency, especially in compressive designs.
Increased resistance to corrosion is secured through the use of alclad alloys or anodic coatings. The exterior or aircraft exposed to saltwater environment is usually fabricated from clad alloys. Anodized bare stock successfully resists corrosion when only occasional exposure to saltwater is encountered. Corrosion resistance can further be enhanced by organic finishes or other protective coatings..."

[Mel]

Originally Posted by rocketman1988

To say that 2024 aluminum is "VERY corrosive" may be a bit of an embellishment...
Is the 2XXX series more prone to corrosion than other alloys? Yes, but you have to put that in perspective.

When I said that 2024 is very corrosive, I was implying that it is more corrosive than other aircraft aluminums such as 6061. 6061 is not quite as strong as 2024, but it is much more corrosive resistant.

[lr172]

Originally Posted by Mel

2024 aluminum itself is VERY corrosive. That's why it is normally alclad. I REPEAT....Do NOT remove the alclad!

If this were true, all of the unprimed kits would be rotting to pieces, as there are MANY parts of the plane made from non-clad 2024. It's true that 2024 is less corrossion resistant than pure Al. It is inappropriate to say that it is VERY corrossive. This simply isn't true. Leave a piece of 4130 outside for the summer next to a piece of 2024. Night and day difference in corrossion. It would be fair to say the 4130 is very corrossive. My 10 kit is 13 years old and not a spec of visible corrossion on any 2024 part.

Larry

[Mel]

Originally Posted by lr172

If this were true, all of the unprimed kits would be rotting to pieces, as there are MANY parts of the plane made from non-clad 2024. It's true that 2024 is less corrossion resistant than pure Al. It is inappropriate to say that it is VERY corrossive. This simply isn't true. Leave a piece of 4130 outside for the summer next to a piece of 2024. Night and day difference in corrossion. It would be fair to say the 4130 is very corrossive. My 10 kit is 13 years old and not a spec of visible corrossion on any 2024 part.
Larry

There are ver few, if any, unclad 2024 parts in the RV kits. The unclad parts are 6061.
Your comparison of aluminum to steel is not exactly apples to apples.

[rocketman1988]

...but comparison of steel, which is very susceptible to corrosion, and aluminum which is not very susceptible to corrosion, is a good illustration of perspective.

[Mel]

OK, My original post was to recommend that you not sand off the alclad before priming.
If you guys want to do that, go right ahead. I'm out-a-here.

[GPV]

Originally Posted by tgmillso

To Greg....
Good luck.

Thanks Tom that was some really sound advice and I think you have sold me on the superetch. I went back to my test part and did a few more scratch tests and was unable to reproduce my previous results with plastic, except in places I had perhaps gone a bit thick on the coats. So I'm reasonably convinced it will cling on for a few years.

Looking at the valspar data sheet you linked I see the product does not need top coating either, and as you say at least meets some form of corrosion standards. I think I'm reasonably happy to proceed with it. Very glad to be here as the prospect of mixing 2 pack and cleaning a gun all the time was getting very discouraging.

I have a 3M respirator also and note all of your suggestions.

Thanks again,

Greg

[lr172]

Originally Posted by Mel

OK, My original post was to recommend that you not sand off the alclad before priming.
If you guys want to do that, go right ahead. I'm out-a-here.

The discussion was around sanding for the application of a sealing primer, which provides the same corrosion protection as the aluminum cladding and in many cases does so more effectively. What is your reasoning for preserving the alum cladding in this case? I suppose it is a good backup in case the primer chips off, but can't think of any other reasons.

Larry