Filler for Nitrate Dope

[NickPasch]

A couple weeks ago, a builder described his method of finishing a wood boat, using nitrate dope to glue down 0.6 oz glass cloth, and then using a filler, primer, and epoxy paint. I am wondering what filler is being used? I wouldn't expect a buterate dope filler to work, but I don't know what else to use. As always, any help is much appreciated.

Nick

[AndyKunz]

Butyrate fillers don't sand as nicely.

I use nitrate dope with corn starch or talc as a filler. I buy either HS (high solids) nitrate from the Piper shop at the local airport, or from Aircraft Spruce & Specialties (another source for wood, BTW). In a pinch I'll get Sig.

Piper is cheapest source locally, then AS&S, then LHS. I get it in quart or half gallon sizes.

For thinner I use 90% acetone and 10% toluene.

Andy

[NickPasch]

Thanks Andy, AS&S it is. Since I live in the San Francisco bay area, it is a no brainer. The Piper rep in our area hasn't seen a tube and fabric airplane in recorded history, or so he says. Considering the humidity, maybe that is a good thing. The shrinking nitrate dope, with reducer to handle the perpetual high humidity (did I mention I can see the Pacific from my house?) looks like a winner.

By the way, what exactly do you use as a primer and ... epoxy paint? Should have asked the first time, but well, you know how it is.

Thanks again,
Nick

[Doug Forrester]

Nick
I also use nitrate clear dope to adhere fabrics and tissues to balsa. A favorite balsa topcoat technique of mine that give the balsa surface an extremely lightweight and moderately tough hardskin using "silkspan" and dope. I can hear the questions already. Andy will know what I'm talking about. We stick and tissus model airplane builders grew up on these materials. Silkspan is a non-woven fibrous tissue paper that is available in several weights and will shrink after it is applied wet.
The technique is to apply it wet or damp and glue it only around the edges of the surface. This gluing can be done with clear dope or thinned white glue. When the tissue dries, it shrinks drum-tight to the surface and then you can coat the whole surface with dope (or even thin CA).

Silkspan should be available from better hobby shops that cater to the airplane crowd and is sold by SIG.

[Doug Forrester]

Nick
I got so involved in talking about dope and tissue (almost a religion to us old airplane types) that I forgot that the subject was fillers for dope. Almost any very fine lightweight powder will work. Corn starch or talcum powder are perhaps the most common. Don't overlook the use of balsa dust too. Modern fillers such as Cab o sil, glass and phenolic micro balloons should be explored as well.

[AndyKunz]

By the way, what exactly do you use as a primer and ... epoxy paint? Should have asked the first time, but well, you know how it is.

The best-all-around primer I have found is the gray automotive stuff. It has lots of solids and cleans up with acetone. It can be brushed or sprayed, sands well, and bonds well to most finishes.

It needs to have a slightly rough finish. Don't try to sand it down to a glass-smooth surface, then apply color. It won't last long - probably just until you go to remove the masking tape for your way-cool paint job. It you leave it with a slight texture, the following color coats will bond better.

The more you leave on the model, the more it weighs, so keep that in mind!

I use SuperPoxy for my epoxy finishes. It sprays well and you can usually lay down two or three coats in a session.

Andy

[NickPasch]

I have taken the advices from here and decided to use heavy-weight silkspan and nitrate dope for my Dumas-derived 33" PT boat. Derived, because every piece of Mahaogony Ply is now 1/8" Lite-Ply and every piece of 1/16" Mahogony verneer is now 3/32" medium balsa. Making a full court press on reducing weight. Deck is 1/32" ply, instead of 1/16". Very light hull thus far.

Except for the bow-20% station, I very successfully used dope and silkspan. 60% dope in two coats for a base layer with overnight to dry between coats, then stick the dry tissue down, working dope out from the center of the piece. Looked kind of nasty, bubbly, but shrank down to a very nice finish. Bow-20% got 0.6 oz glass, similarly stuck down with dope because I was having so much trouble getting the silkspan to do the compound curve thing. Dope and glass is god over compound curves. A couple of coats of 90% dope and time to fill. Had some troube sticking down the overlapping silkspan tabs at the edges until I remembered that a 50-50 mixture of dope and Amberoid, with a small amount of dope thinner to make it easier to brush is a great tab sticker downer. Paint the wood surface and then push down the tab and wipe. Worked great.

Filled the glass and silkspan with Hobbico Balsa Filler. Very nice, but it is important to wait long enough for it to fully dry, sometimes overnight, before sanding it back. Otherwise, not a very flat surface will result. If you sand and there is only powder it is dry enough, clay-like balls means it is not dry enough. I alternated Balsa Filler and 60% dope coats and the glass fulled up pretty quickly (3-4 coats). To make the final smoothing I used equal volume mixture of 60% dope and cornstarch with lots of stirring. I gave the whole boat a normal coat and then before it was dry ( < 5 mins) I gave the bow a second thicker coat. Disaster! The bow surface cracked all over the place where I gave it the second coat. Several coats of fill and dope and it is now pretty good again. It seems like the excess solvent of two coats close together is very bad.

Deck got a medium silkspan and dope coat and it worked great.

In the future I will try to use silkspan strips to cover the bow, since filling the silkspan is so much easier than filling the glass.

Thanks for all you help and advice guys.

Nick

[Doug Forrester]

Nick
Sounds excellent! In future, when faced with compound curves and silkspan, Try slitting the silkspan. Also don't be afraid to overlap it. Real tight curves are best done , as you intend to try, with strips of overlapping tissue. After it's doped, it's fairly easy to sand the overlaps flush. If you are having some trouble adhering the tissue , try thinned white glue or aliphatic resin (the yellow version of carpenters glue - Titebond). After it's all dry you can dope over it.
Keep us posted of your results and things you've discovered.

Thanks

[NickPasch]

One thing that I have noticed when filling, I tend to oversand the area and need to fill again. What is sad is if I oversand the first time, I am just as likely to do it a second time and have to refilled a /third/ time! I finally figured out that if I have an area that is oversanded, that I should paint it with 60% dope before refilling it. The dope harderns the filler and I am much less likely to oversand the fill that follows. If I use dope after every fill layer (assuming that the filler is below the reference level) it is much simpler to come to reference.

I suppose everone knew this but me, but it really does help.

[toyboy4145]

I dont know if this applies but, Ialways spray a lite coat of primer on top of the filler. It seems to help me identify where needs sanding and where doesnt.(I'm heavy handed with sanding). Just something that I found that works for me.

[AndyKunz]

I like to use silver or primer for doing this.

Thanks for sharing this - we all need to be reminded from time to time what we take for granted!

Andy