lml@lancaironline.net Mailing List Archive

I think one of the conditions that puts cracks in paint is when you have several base materials in close proximity.

I often see cracks around windows, especially windows installed to an exterior joggle. No surprise, since in that area the window material, adhesive, cloth/resin, filler, primer, and paint, all reside within a few thousandths of an inch from each other. As the mix heats and cools, each material expands and contracts at a different rate, vibrates at a different frequency, flexes differently, etc. Tough duty for the paint.

One investment that seems to protect the paint from cracks is a single layer of very fine cloth (ACS P/N 1080-50) on any seams, potted areas, bodywork, joints, gaps, whatever. Then the paint only has to flex against a single, unitary base.

Regarding being able to see the weave--I eventually just started thinking of it as proof that you were dedicated to keeping it light!

Dave Saylor
831-750-0284 CL

On Mon, Dec 16, 2013 at 9:17 AM, Michael Newman <mnewman@dragonnorth.com> wrote:

I had cracks extending from the corners of the door hinges. They sound similar to Bob's. They were thoroughly investigated as part of the structural repairs now nearing completion on N776CM. Because there was other working going on to replace the windows we were able to grind into them fairly deep and make sure nothing was going on.

My opinion is that these are caused by pressure flexing and that there is nothing you can do about them.

On Mon, Dec 16, 2013 at 9:33 AM, Robert R Pastusek <rpastusek@htii.com> wrote:

Justin Wrote:

I recently had my ESP painted and there are already significant cracks showing in the paint around the door frame corners and hinges. I was told that the flexibility of modern urethane paints would prevent this. How common is this for the pressurized aircraft?

Justin,

I went to great pains to prevent this while building my IV-P…bedded every through-bolt in flox/epoxy, and covered every through-bolt around the door and over each “chicken plate” where the pressure bulkheads attach to the fuselage (except two that I missed…but that’s another story). Anyway, N437RP looked perfect when we first flew it…and for about a year. Then I started noticing very slight “dimples” where the screw heads were located. No cracks or breaks, but the surface was being pulled inward very slightly…I presume by flexing during pressurization. To be sure, these are very difficult to see; I’d likely need to point them out with a light or specific sun angle for you to see them at all, but they are noticeable to a discerning builder…

At almost 6 years/1200 hours now, I am starting to develop slight cracks in the paint around the door latches. Nothing longer than ¼,” but every latch attachment as at least one. They are more noticeable than the dimples. I am sure these are not structural defects, and I took one of the latches off the fuselage to look at the flox pad it’s mounted on…just to be sure. It’s absolutely solid; no cracks or visible defects. My opinion is that the fuselage/door undergo a lot more flexing/expansion than you’d imagine during pressurization cycles; that some deformation/cracking at the stress points is inevitable. …one reason to be sure the cabin over-pressure relief works as intended… J

I’d be interested in the comments of others, but I’m not sure how/if these can be completely eliminated.

Bob Pastusek