Great Planes J3 Cub

Jackson T

Elite member
If anythings for sure don't do the method that I described in the quoted post :p it ended up being a waste of time. Vacuum bagging is really the way to go as it gives you very smooth parts needing minimal extra work to finish as well as giving you very good adhesion between layers.

I make all my molds out of Ultracal. I'm not sure if you can get it in Australia without incurring some ridiculous shipping fees but if you want to try using cornice cement I would test it on a small scale for minimal material waste. Building type cements tend to be a poor choice as they're usually meant to plug gaps and not adhere to fine details of a given surface as mold making materials are, and they also tend to be more porous, which will affect the quality of the finished part significantly. You can also use plaster of Paris which takes up detail better but the molds tend to be very fragile and generally don't survive making more than one or two sets of parts.

I used to use dedicated release compound supplied by Sig for molds but upon running out of that and discovering that another tiny container of it cost like $10, I tried carnauba wax (the stuff you polish car bodies with) and it works just as well and can be bought locally pretty much anywhere.
Thanks for the tips! I'll see what I can find and let you know how I go.


Legendary member
Cowling is getting close! Today was spent getting the final surface prep done and putting down primer. People say you don't need it, but it works wonders to get a smoother finish. I prefer to apply real heavy coats and then sand a lot of it off. It works much better than most filler materials at filling tiny pinholes and scratches in the part being finished. There are still a few large pinholes left, which I will fill with spackle, but so far I'm happy. I managed to find Rustoleum paint in cub yellow (!) and to my delight it matches the covering very nicely. Just need to fill the pinholes and sand it some more and it will get some color.

The dummy engine has also had some work done to it. I made the exhaust and intake pipes out of balsa dowels. I wanted the look that these pipes usually get on the full scale cubs where they get blackened over time but they're still shiny, and accomplished that by painting them silver first and then going over that with a lighter coat of black paint before the silver dries all the way. The paints will sort of intermingle while they're drying and you get the nice "shiny-but-weathered" look.

Next I'll make the "eyebrow" air scoops. I'm not sure whether to make them out of sheet aluminum or plastic...



Legendary member
A belated update but I realize I never provided one at all, so here goes. This plane has been enjoying many successful flights for the past 4 months. In all that time I have not taken very many decent pictures of it either...


The cowling paint match is about the best i've ever seen from a store bought rattle can. Sometimes I joke that I have the last can of Lustrekote cub yellow in existence :p I've also realized since that time that Rustoleum paint is not entirely glow proof; over time it dulls if exposed to glow exhaust and if not cleaned in due time it gets sticky, but it doesn't just slough off like something like latex would do. Fortunately this engine is very good at not spewing too much oil on the inside of the cowl, so the paint has held up well.

Initially I had a very nice finish on the cowling accomplished by painting it in cub yellow, polishing it, and then clearcoating it and polishing again. After it dulled I could repeat polishing the clearcoat to restore the finish but over time it became less effective for some reason, and I no longer do it. The cowl is still fairly shiny anyway.


I also built the "eyebrow" air scoop for the dummy engine but I guess I entirely forgot to take any pictures whatsoever of the plane with it equipped. It fell off on the first flight with it installed and I was never able to find it, and I never built a replacement, so I guess it doesn't really matter anyway. I also had an idea to build a scale air filter but that was also scrapped; but, I did put a T-fitting on the oil transfer tube and routed a tube through the hole for it so I can put after-run in the engine without having to pull the cowl.

Initially I was opposed to it but this plane is now in line behind the Kadet for float installation. Who's not to like a cub on floats anyway? I think all I'll really need to do with the fuselage is add some reinforcement for the rear float mount and also move the switch off the bottom, because I don't think it and water would get along too well. I might also add a bit of vertical area at the tail if it needs it.


Master member
When I eventually get to my L-4, I hope my dummy engines look as well done as yours. (y). Very convincing metal paint job.