What to do with Skin Scratches

RVG8tor · #1 11-10-2007, 08:17 AM

I am about to prime the inside of the VS skin, trouble is during the dimple process I managed to drag the skin a few times against the DRDT

and just the cleco and un-cleco process seems to have left a few scars. I know leave the blue tape on I did not.

These scars are not removed with a light rough up with a red scotch brite pad. I am priming with SW P60 wash primer. My question is do I use something rougher to get out the scratches or do I just use the scotch brite pads only

My concern is taking all of the alclad off of the surface which is the primary corrosion protection. I am not sure how deep a scratch can cause future cracks if at all but I want to dress them now if required.

If I do use something rougher should I use a heavier primer, like the SW 988 rattle can stuff I use on the ribs?

Thanks for the help!

JonJay · #2 11-10-2007, 08:26 AM

Pictures would help, but you probably have nothing to worry about. You will have a lot of scratches here and there through your build. There may be a technical reference in AC43 some of the experts here can refer to.

lrfrey · #3 11-10-2007, 09:01 AM

Make sure the scratches are "eased", that is no sharp edges. Scotchbrite usually will fix this quickly. Taking alclad off of larger area isn't going to be benificial. As far as the alclad that was removed... that's what the primer is for. If the alclad wasn't scratched it wouldn't need a primer. Use a good primer and move on. (BTW, primer is applied as a light coat, it ain't paint". JMHO.

Larry

RVG8tor · #4 11-10-2007, 09:33 AM

First thanks to all for the advice. I was just thinking, how do you know you have removed the alclad entirely from an area? I know it is not thick but is there a noticeable change in color or some other way to know you have entered the underlying metal and remove all of the alclad? I know this is something you do not want to do, just curious if you can tell.

SvingenB · #5 11-10-2007, 09:49 AM

The main purpose of the cladding in alclad products is to sacrificially protect the core (somewhat opposite of what can be read in some places). To even be called an alclad product the cladding material has to be anodic to the core, typically by 80 to 150 mV. In a corrosive environment the cladding will always be corroded first. The corrosion will only spread down to the interface of the core/clad, then it goes out laterally without touching the core. In this way it protects the core at edges and around holes. Likevise it will electrochemically protect the core at scratches and other damages that goes through outher layer.

Unarguably the cladding will also protect the core because it is made of an alloy that easely formes a strong oxidation layer when in contact with air, but that is not the main purpose of alclad.

A great book on the subject is "Corrosion of Aluminum and Aluminum Alloys" by ASM International.

morlino · #6 11-10-2007, 10:33 AM

Quote:

Originally Posted by RVG8tor

First thanks to all for the advice. I was just thinking, how do you know you have removed the alclad entirely from an area? I know it is not thick but is there a noticeable change in color or some other way to know you have entered the underlying metal and remove all of the alclad? I know this is something you do not want to do, just curious if you can tell.

I do not believe that you can visually distinguish the difference between the layers without etching and magnification. I remember reading somewhere that the pure aluminum layer of alclad is 10% of the total thickness. I interpret this to mean 5% per side, so 0.00125" for a 0.025" sheet. Just for comparison sake, I measured a piece of heavy duty aluminum foil from the kitchen with a micrometer and it was 0.0014". Not exactly thick but not exactly thin when you think about how long it would take to turn it into dust with a piece of red scotchbrite.

As others mentioned, priming should provide adequate corrosion protection and you want to make sure that the edges of the scratch are smooth. Pay extra attention to scratches that cross the edge of a sheet or the edge of a rivet hole, these are going to be potential start points for cracks.

Jeff R · #7 11-10-2007, 06:03 PM

I just hit any areas where the primer had been scraped away with some rattle can zinc chromate primer. Deep scratches should be sanded smooth to prevent stress cracking and stress corrosion.