What's the best way to finish a balsa covered model?

[CWSparky]

I have a Sig Kougar that is 90% complete. It had been unsuccessfully painted with some sort of epoxy paint by the previous owner. I plan to sand this off with progressively fine sandpaper and re-finish the model. I would appreciate some advice from experienced builders on the best method of refinishing the plane. The Kougar has a balsa covered foam wing so I understand any of the heat activated coverings could melt the foam, ie Monocoat, Ultracoat etc. Also, I have no experience applying these types of finishes although I have done silkspan/tissue and dope in the past. I know the traditional dope finish is an option but I’m wondering what other easily available finishes would provide a good looking fuel proof finish.
Thanks!

[cgroves]

There are two techniques I've used in the past for Balsa over foam. The first one is Monokote. Unless your iron is ridiculously hot you won't damage the foam underneath. You really only want to get the covering material warm enough to activate the glue and you shouldn't keep the iron in one spot for very long.
The other approach is to glass the wings with a really thin layer of glass/epoxy. This can give a really nice finish but takes a lot more effort than monocote and is heavier (but not much if it is done right). It is also handy for adding some strength without to much weight.

[Tom Hastie]

Go with Monokote or Ultracoat. Most balsa foam wings are covered with iron on film these days. It's the simplest, cleanest and least stinky method.

Here's an example of a foam wing I covered in monokote last winter:
http://www.calmdays.com/showpost.php...&postcount=161

You will end up melting holes in the covering long before the heat makes it to the foam inside the wing. (Balsa is mostly air, and therefore a really good thermal insulator).

I have even covered bare foam with monokote with good results. At the time I was making a set of floats for a small electric model. The monokote provided most of the structural strength for the floats and they ended up working really well. Here I did have to be a little careful with the heat, but I found it was easy to stick the monokote down without warping the floats.

Applying the iron on covering takes some practice. Do some reading online there are many good tutorials on the different RC Forums.

Tom

[CWSparky]

Thanks for all the tips! The controll surfaces are already installed (hinged and glued) Will it be a problem for film coatings if I just cut around the rudder, ailerons and elevator and bond to the wood as best as I can?

[Tom Hastie]

Covering before hinging is much easier. So if at all possible, I do it that way.

If the surface is already hinged, I'd do it this way:

1. Cut a ~1" wide strip that is the same length as the distance between two hinges.
2. Put the strip into the hinge line between two hinges. Deflect the control surface as much as possible and iron it down on one side.
3. Deflect the control surface the other way and iron it down on the other side.
4. Repeat for the space between each hinge.
5. When the part in the gap has been covered on both the wing and the aileron, Now you're ready to cover the top and bottom of the wing.

This isn't the only way to do it, but it'd probably be the way I would go. I find it much easier to hide the seam at the trailing edge of the wing rather than trying to iron the seam into the hinge gap.

Hope this makes at least a little sense.

Tom

[John Weekes]

Given your circumstances....that the model has been painted...film is your best option.

My strong preference is Ultracote for easy of application. Monokote works too...but I find Ultracote to be pretty much "simple-stupid" to use...very predictable and easy to apply...especially when the application gets at all complicated.

I'll second Tom's technique for applying film when the hinges are already installed.

IF the hinges are the "wick" type (solid pieced of plastic without a metal pin)...you might be able to cut them off...trim them flush...and re-hinge. I've done this before. It gets a little tricky...and presumably the new hinge locations are not ideal...the original ones were strategically located in relation to air forces and stress.

Given that the re-hinge option is a little more complicated, go with Tom's recommended method.

I think I spoke with you recently about this at the ORCC field. I'll say again for the potential benefit of other readers...

1) Ultracote (clearly my bias);
2) 21st Century heat iron with accompanying sock (other irons are crap);
3) Decent heat gun...most types are fine...mine is a Hangar 9, I think;
4) Sharp scissors
5) Metal straight edge
6) Sharp #11 blades...keep changing them out...they go dull very fast...I buy packs of 100...but then I do a lot of this...

IMPORTANT: Follow the film manufacturers recommended heat settings for base application over solid surfaces (vs. open structures) and for shrinking. The Kougar is solid...application and shrinking is a little different than covering an open wing structure...(not more difficult just different). All films types give off gases...so does balsa (very porous)...film will bubble up very quickly if the iron it too hot over wood or over itself (when you are overlapping pieces). ...It will also pull back/shrink away when you are overlapping and the iron is too hot. ...This is very frustrating when the pieces are different colours and curved, etc. ...Of course you can hide this with a trim piece of yet another colour...but it is frustrating. ...I guarantee this will happen to you...it has happened to all of us.

Keep the iron and gun away from plastic structures...I believe the Koug has a moulded plastic turtle deck behind the cockpit. It will melt. Best to paint the plastic.

Keep your colour scheme basic...but high visibility and different on the top and bottom wings. Faye Stilley is the God of film application and only uses Monokote...google some images of his models...unbelievable.

Be prepared to muck things up a bit.

Hope this helps...just a few pointers having covered 30-40 planes over the last 30 years or so...

John

PS: The Kougar is an excellent flying model...very predictable and a decent entry level aerobatic machine. ...An "oldie but a goodie". Have fun with it! I regret the day mine bought the farm.

[CWSparky]

Thanks again Tom for the additional tips and John too. Yes John, I talked to you at the field a couple of weekends ago. I think I have enough info now to tackle the job over the winter. The hinges are the plastic pin type and seem to be glued on solidly and caked up with paint so I'll try to work around then as best as I can. I picked up a video on applying Monocoat so that was helpful as well, however the demo was using a newly constructed model, so no paint of unknown origin underneath.
Rod