I'm Spitting Mad and An Observation on PPG Omni

WEll...the new Katana is being worked up to full competition mode now. It has been out in the sun for maybe 10 hours total if that.

On the metallic green leading edge paint area, (just the top side) there are a series of "blisters" and grain lines rising from the blocked smooth finish. Right now, and until I decide to sand them down and refinsh this part of the wing, I cant tell what part of the system is failing, although I suspect it is the Polycrylic system. I suspect this since from there on out, it is the same finish I had on the SV-40 and it exhibited no such problem--even with the same and darker colors.

Anyone else see this on a PC finished wing? I used light silkspan, PC and PC talc--block sanded dead smooth--320 finish then to Gray primer--semi-opaque--then PPG Omni--then Matrix clear.

Second: After crashing the SV-40 at the Nats I had the opportunity to look at pieces of the torn covering on open bays. THis was done with medium silkspan, Nitrate, talc/nitrate, grey primer, Omni BASE/Clear.

Anyway, this plane is about 1 year old. I had observed cracking of the finish around the edges of the center sheeting, and a couple of rib caps. It wasn't clear through, just the top layers.

After examining the removed pieces, I see that the open bay covering and paint is very brittle, breaking clean through like a piece of UV embrittled acrylic.

Anyone else see the same? ANy experience with the plastic bumper paint additives that make auto finishes more flexible? IS the Auto system perhaps not a good solution on open bay wings where surface flexing is a problem?

Ideas?

Thanks

LAST EDITED ON Aug-17-04 AT 08:31 AM (CDT)
 
Curt,

I meant to talk to you about the grain showing on your wing but I got busy. I don't think its the fault of the product as much as it is prep. When I prepare the wood for finishing the last thing I do it to give the entire model a "bath". I wipe the entire model down with a fairly wet cloth and swell out all the areas that where compressed during sanding. When its dry I go over it very lightly with 400 wet & dry taking care not to recompress any grain. You'll notice that the majority of the problem is on large sheeted areas and I would imagine you presanded your sheeting before putting them on the wing. Flat block sanding is good, but too much pressure will cause grain compression that will show up later when the wood is heated in the sun.

I can't address the brittle question.

LAST EDITED ON Aug-17-04 AT 09:12 AM (CDT)
 
Hi Randy:

Well yes I did flat sand the panels prior to putting them on the wing foam. Then, I also do the wet rag trick (old wood workers thing) and very lightly take the fuzz and swelled highs off after it is dry--320 or 400.

But you're right--it looks just like what you described except there are raised "warts" that are clearly not related to the grain stucture.

I guess I'll know better after I knock it all down again after the season is over.

The other thing is, with the PC being a water-based material, I would have thought that any grain raising would have taken place at the point of covering with the first coat.

Thanks Randy

Curt,

My current plane is a dopeless finish including the open bays. All Polycrylic from the wood up to the K-36 primer then urethane color and catalyzed polyurethane clearcoat. I'm having similar problems with grain showing and blisters. The molded balsa nose top block is just a carpet of grain-wise very small blisters and grain showing. I think it may be the water based Polycrylic or some combination of the Polycrylic and K-36 primer. I also didn't have a problem until the plane sat in the sun for a bit. The bottom of the plane still looks like it did when I built it, so that lends some credence to the idea that it has to do with some interaction between the finish materials and direct sunlight.

So, lesson learned, I guess. The new classic plane is non-taunting Randolph's from the wood up to color (filler is Randolph’s clear and Aero-1). I plan to use (in the next couple of days) urethane auto paint color and clear will be Matrix with a flex additive. Testing I've done seems to indicate that this will hold up pretty well. I did a 6"x12" sheet of 1/16" balsa this way from bare wood to clearcoat and left it on a table in the yard in the direct sunlight (about 85 degrees) for most of a day. I used 4 colors from white to yellow to dark metallic red to black. On inspection, there was no change that I could see.

I guess we’ll see.

>cracking of the finish around the edges of the center
>sheeting, and a couple of rib caps. It wasn't clear
>through, just the top layers.

The exactly same thing happened to me do to my stupidity. I sprayed a 2 part epoxy primer over clear coated polyspan on the wing open bays and then sprayed Sig dope over that. Primer cracked from flexing on the open bays, it's has no flex to it, but the polyspan remained good. Lesson learned

ANy experience with the plastic
>bumper paint additives that make auto finishes more
>flexible? IS the Auto system perhaps not a good solution on
>open bay wings where surface flexing is a problem?

Doug Moon's planes are some more than 4-5 years old with more than 1000 flights on them. They are open bay. No cracks anywhere.

The bumper additive is a fine solution, although I think it is overkill. Bill Wilson has discussed its use. It is certainly an alternative.

'zilla,

I think that occasionally the catalyzed clearcoats can get a bit brittle if exposed to harse sunlight over a period of days. While not a real problem for cars (it being painted on metal, after all), it can be a problem for finishes required to flex...at least some. It isn't always true and some of it (the brittleness) may be due to conditions at time of mix (cold, hot, humid, whatever), amount, if any, of reducer added and frankly, quality of the product used. The flexible panel additive is certainly a "fix" for this.

Part of my problem with the molded nose block was that it is only 1/16" sheet. Now, it's fiberglassed with West Systems epoxy and half oz. glass on the inside and has CF veil on the outside, but it's still too thin and probably contributes to paint breakdown since it flexes a lot (too much). Another lesson learned. But it is also present to a lesser degree on spots all over the upper surfaces. And I think it may have to do with some interaction with the PC and the primer. Never had the problem before and the stuff above may be the primary culprit, but it may be that using the water based PC in the rather severely humid conditions I had when working on this may also be a prime contributor.

Curt,
I didn`t know you crashed your plane at the Nats.Sorry to hear that.Can you tell me what happened and how bad was it?

Gordy

WEll..it was a brain fart I guess. As you know, though a combination of fortuitious circumstances, and the fact that I flew each flight, I made it through to Friday. First flight that morning ws ho hum--nerves and all...

So my second flight I felt I had alot left to give the judges. THe weather was really perfect...I was really thinking about what I was doing...very nice(for me) take-off and level. great entry to the wing-over...At about 12:00 o'clock in the wingover, I realized I could not remember if it was the first pass or the second. My observer said the plane actually bobled as I began the pull-out upright then I came to and went the right way..inverted.

IfI would have been doing one of my normal 10' inverted segments, I would have been fine, but I pulled out at about -6".

Plane is pretty torn up but I might re-do it just as a smaller practice plane. The motor survived fine.

I had'nt lost a plane in the three years going from beginner to advanced so maybe I was due. Anyway, In hindsight, I think the excitement and "pressure" of the whole week got me to thinking too hard about what I was doing. I'm better now!

THanks for asking. Also sorry to have seen your mishap..I'm watching with great interest your re-build progress.

The loss of the SV-40 forced me to jump to the Katana which was standing by in the wings just unflown as the Nats began--its really coming along well.

Regards Gordon

See ya in the spring.