Finishing - Sanding

Help!
I saw an example of silver coating for finding finishing flaws once and I'm trying to match my work to what it's supposed to look like when finished. I've got 1/2 dozen coats of dope on my plane and a base coat of white on the wood - I'm sanding with 400 and 600 paper to get that smooth consistent finish. What I see after sanding and holding the plane up to the light as I go is that there is a "speckle" pattern of glossy areas and dull areas. The more I sand the lesser the dull areas become and the greater the shiny. If I'm doing this correctly will the whole surface appear glossy when I am finished? What exactly should I be looking for and what's the technique for seeing the results?

Any surface not sanded perfectly level will show ripples or uneven light reflections. The technique is called "candling", which is looking at the surface in a darkened room against a light source. Jeff Troy describes this well in his book "Finishing and Detailing Model Airplanes". It's quite good, I'd advise you to get a copy. Basically what you need to do is build up the low spots with filler coat, and block sand until you get an even sheen on the entire surface. It should like a piece of glass in other words. I usually spray on the silver, use filler coat and sand until I think it looks ok, then sand some more, and spray on another coat of silver. Repeat until the surface is smooth with no low or high spots anywhere, THEN start your final color coats.

Once upon a time, we used to buy an aluminum powder to add to dope because there wasn't much "silver" in the dopes available at the time. I think Windy was one of the sources, if you couldn't find it at your local artists' supply outlets.

I'm curious about the quantity of silver "solids" in the latest dopes, the Randolph mixes that Brodak sells. Do any of you folks recall if Windy still adds more silver? My newest how-to tape from him goes back quite a good while now -- older than the dope formulas from Randolph!

Kiwi

Aluminum paste added to clear dope is a full-scale aircraft finishing step dating back to the earliest days of aviation. Randolph has been a supplier to the full-scale aviation industry for decades and still produces aluminum paste. They dont sell direct, but one online source for all Randolph products is found at:

http://www.texair.com/randolph.htm

One thing to remember: all metal powders used as pigment reduce adhesion of subsequent overcoats, so when mixing your own aluminum paint you must keep the amount of pigment to the minimum necessary for coverage.

Bill

Thanks!
That is exactly what i'm doing! Albeit with white as my base color rather than silver. Jeff Troy is a good guy - flew with him a couple of times when I was still in Philly. I'm gonna apply some more filler tonight to see what happens.

The reason for using silver rather than white as a basecoat is that it shows the smallest imperfections better than any other color (MUCH better than white!), save prehaps gold. It is also much lighter than white, an important consideration. Next time, use the silver, and if white is your main final color, you'll find you'll use less of it to acheive a nice finish.

White pigment is said to be Titanium Oxide, and is supposed to be the heaviest. I recall Windy writing that he put the "silver" pigment (probably aluminum?) into clear dope, and I believe it was SIG Lite Coat, the low shrink stuff. Regular silver dope is not supposed to adhere well, and will peel up pretty easily with masking tape. You are ahead of me on the learning curve, but that is what I remember from Stunt News. A lot of guys are using Polar Grey now instead of silver, because the adhesion is better. G'luck, Steve

If you're talking Sig butyrate, that I can believe (poor adhesion). I've had the white flake or peel off when the tape was pulled (drafting masking tape too, less tack), and I no longer use Sig dope. I make my own silver, just put aluminum powder in clear.

>dope. I make my own
>silver, just put aluminum powder
>in clear.

John;

What is your source for the aluminum powder? Also, what size is the powdered aluminum, e.g. the aluminum is powdered down to "x" microns.

Bob,

I am still using up a large jar I got from my late father many years ago when he was with W.H. Curtain, a scientific supply house. They are Fisher now, and I suspect it is in the chemicals catalog still. The stuff is so finely ground, it will actually "float" in the air as you open the lid, and lightly brushing your finger tip across the inside of the rim will leave it a nice shiney silver color, almost like chrome plate.
I think the metallic pigments you can get from any good art supply store (artists use them for metallic effects) are as fine. They come in a BUNCH of different shades too. If you want bronze or gold dope, that is the way to go. I get mine from Texas Art Supply. I think they mail order, if you don't have a similar outfit near where you are. They have the acrylic dye you can use to tint dope for translucent or "candy apple" effects, also.

J

>Bob,
>
>I am still using up a
>large jar I got from
>my late father many years
>ago when he was with
>W.H. Curtain, a scientific supply
>house. They are Fisher now,
>and I suspect it is
>in the chemicals catalog still.
>The stuff is so finely
>ground, it will actually "float"
>in the air as you
>open the lid, and lightly
>brushing your finger tip across
>the inside of the rim
>will leave it a nice
>shiney silver color, almost like
>chrome plate.

Just a word of caution with fine metallic powders. They can be highly explosive in a suspension with air.

>What is your source for the
>aluminum powder?
>
>Regards - Bob Kruger

I got mine at the local airport repair department. If you have a local A&E who does fabric, he should have some. We have a college in Vincennes that teaches mechanics (at the local airport). It is probably a lot cheaper here than at a craft shop.

The Brodak Polar Gray works well enough I have not been tempted by the perfectionists' silver. Certainly no adhesion problems, whether Brodak color or a shift to Krylon. Caveat is that I have not yet tried for a front row finish, just a good one.

Steve,
I here you about the tape pulling up! I'm going to try taping tonight. The candling really worked out well and next time I'll use Polar gray for the finish. I also have auto primer I might try as well. The reason why I used white was because it was originally the finished color of the plane. With a season behind me on the plane (a flite streak) and some damaged repaired I decided to try for a better finish. Six months later I'm getting the final coat on and trimming the plane out. Hopefully I'll fly it before winter.

"They have the acrylic dye you can use to tint dope for translucent or "candy apple" effects, also." JAK

I think you must mean something other than acrylic. Acrylic is a plastic, the powder of which is used as pigment. Maybe you were thinking of Analine Dye? This is a good piece of information if this is the case.

"Just a word of caution with fine metallic powders. They can be highly explosive in a suspension with air." Brett

Guys, do not overlook Brett's brief message or forget it. Powdered metal of almost any kind can explode under the right circumstances. Same for wheat flour, and I would think almost any kind of fine powder. Caution is always in order. Do not volunteer for the Annual Darwin Awards! Steve

No Sir, I mean liquid acrylic dye exactly as I said. It is indeed a plastic, and compatible in not only polyester resin, but epoxies, and dopes as well. George Aldrich clued me in to to stuff a while back.
The analines are HIGHLY toxic, this stuff is not. Try it sometime.