Finishing a composite airplane

[George Sychrovsky]

I wrote a page on finishing a composite airplane, Some people facing the task might find it helpful.

 

http://curedcomposites.netfirms.com/finish.html

[Jerry Schneider]

One of the most helpful, informative posts to date!!!

 

Thank you! Thank you! Thank you!

[Marc Zeitlin]

Originally posted by George Sychrovsky

I wrote a page on finishing a composite airplane, Some people facing the task might find it helpful.

 

http://curedcomposites.netfirms.com/finish.html

There are many ways of finishing composite aircraft. The only part of this that I would take exception to is the last section where you recommend using polyester glazing putty. These putties will shrink, and can cause imperfections later on in the painted surface. They also do not have as good adhesion as do the epoxies. There's no reason (especially if you use the "Smooth Prime" primer) to need or use these glazing putties.

[John Slade]

Very nicely done, George.

I wouldn't argue with anything you said there, but I might be able to add a couple of points.....

 

A heat gun helps during the application of very dry micro. I used hot (115F) MGS epoxy and added micro until the mixture was very dry and flakey - ie it would not stay in a ball or lump. It helps if the ambient temp is in the 80's. Warming the parts with a heat gun also helps. Sanding should be done within 24hrs, before the micro has had a chance to complete its full cure. Don't worry about getting the surface completely smooth - 33grit paper will level the bumps in seconds. In the case of something really tricky like a cowling, dont worry at all about the surface consistency, just ensure that the entire surface is covered with micro. I found that applying micro with a gloved hand worked well for tight corners.

 

I also tried glazing putty or similar for the very tiny nicks. I liked it. I understand Marc's concern with respect to dips and bumps, but the areas I'm talking about filling here are TINY - a couple of thou thick. Less than the thickness of a coat of paint. Even if the putty shrunk 20% I dont think you wouldn't see it. Use it as a primer filler and you might have problems.

[unick3]

Great post!

 

I did a very similar technique for finishing my wing with a few exceptions....

 

1. I pre-marked the low spots by using a 4 ft ruler laid across the wing at different angles. Using a marker outline the dips for the heavier fill.

 

2. Using a slightly thinner mixture of micro, with a 6 in trowel to put a REAL thin coat of micro on the wing.

 

3. After 3 hrs (West System) while the epoxy was just still a bit soft, mix up a very dry mixture of micro. Apply with a 12 in trowel. The outlines of your heavy fill areas will show through the first coat, so build these areas up. DON'T worry about getting a perfect surface. Missed spots, trowel ridges, holes, are OK because you catch them in the third coat.

** the first coat of micro helps keep the second "very dry" micro coat from rolling off the surface.

 

4. After 4 hrs use a 4 ft sanding board and JUST do a rough cut of the surface. The object is to get any really high spots off (as well as bumps, drips, ridges, etc). You want a relatively smooth top surface (leaving all the dips, holes, etc).

 

5. Lastly, using a dry micro, fill in the holes, dips, bad spots left from the second coat. I pulled the trowel really tight to the surfaces as the object is to fill in low spots and not to build up thickness. You will end up with a surface that is really smooth and of relatively consistent thickness.

 

** This technique is similar to finishing a plaster wall... First a base coat, a scratch coat and a top coat.

 

6. After about 1 day cure, I used the 4 ft sanding board with a coarse sandpaper to take the micro down to JUST above the surface of the glass. The micro sand REAL easy as it has not fully hardened.

 

7. Let the micro cure for a few days and sand with your fine sandpaper to your finished height.

 

This process sounds like a bit of work....but... my first wing using the traditional fill, sand, fill, sand, etc took me weeks to finish. My second wing tool about 3 days per side. The micro was easy to sand (no hard spots), and I could sand it all at once with NO refilling.

 

I really like the idea of spraying the place first with a black epoxy primer to give you an idea of when you are getting close to the glass.

 

Lastly, POST CURE your wings after finishing. If you don't, you WILL get some print though in about a year or so. I can easily show anyone on my plane where I post cured and where I did not. It is an easy step, easily forgot...

[GregL]

I've starting a discussion/thread in the Cozy Email list about the finish color of white as a choice. So far it's been generating some enlightening discussion. I'd like the general canard community to comment on the use of other colors for the final one. In fact the only non white canard plane I've seen (online) was a yellow Long EZ. The main reason for white is a temperature affect on composite materials. My question is that an absolute we all have to accept or are there other "safe" choices. I'd like to hear from those who braved other colors. Rick Maddy post a color/temperature chart on his website. Take a look. http://www.maddyhome.com/cozy/chapter?c=25&s=5

[LargePrime]

a question about the paint.

 

In the artical it was recommended that a black plaint be applied to make spotting filling easy. Is there a type of black paint that should be used or avoided? It would seem that inappropiate paint might create an unbonded layer or otherwise cause issues later on. Are there any recomendations or cautions?

[John Slade]

Actually any color other than white will do fine. It's not a "coat" it's a fine mist. I used an automotive enamel spray can. You sand it all off anyway, so I dont think the type matters so long as its not silicone based.

[LargePrime]

Is not the purpose to see when you have sanded through it at the high spots?

Does this not mean you leave it, with a layer of micro over it, at the high spots?

Does this not mean you leave some of it on the plane?

[John Slade]

My approach was to draw a circle around the low spots, then sand the paint away and refill the area. As I said, it's just a fine dust so sanding it away is very easy. I must admit that I didnt find this technique very helpful and didnt use it much.

[George Sychrovsky]

Clarification of the black primer coat -

In following my method as described on my finishing page, This primer coat is applied in a thin but wet coat to insure it bonds to the surface. The primer does stay on the airplane under the micro so it is not sanded off .

This primer coat does not serve as a guide in “spot filling” . In following my method there is no spot filling ( there is only one total fill). The black primer serves as a guide while sanding/contouring the surface. It will give you an indication how much micro is left, the micro is translucent and the black will start showing through progressively turning darker long before you sand through it (see the pictures). Obviously, it needs to be black to give you the highest contrast against the white micro.

It needs to be a primer, not a paint. Primers are designed as intermediate coat so they adhere to the surface below and subsequent coat adheres to them , paints will dry with glossy surface and subsequent coats don’t stick well so you don‘t want to be applying micro over paint.

As to what kind of primer to use, I would assume any primer designed for automotive or marine applications would do fine. It would be a “non sanding primer, the kind that is used just before the final top coat. I use uspaint AWLGRIP® 545 Epoxy Primer-Base D3001. This primer is dark gray and I add black pigment in to it.

Hope this helps

Regards, George

[Don Woodward]

Dear All

 

Question

 

Would it be possable to coloure the (White)dry micro black in order not to have to spray black primer?? carbon microspheres???