Hiding Balsa Grain Under Monokote

[dwells]

Guys, what works best to seal balsa wood grain, harden it a bit and allow Monokote to adhere well? I like the high gloss of Monokote but don't like the way the grain shows underneath.

[IBWALT]

First make sure that the balsa is sanded as smooth as you can get it. Then use your iron to bond your monokote to the balsa around the edges. Once that is done use a hot air gun to shrink the monokote. No need to iron it down over the entire surface of the wing or fuse. This will give you a nice smooth surface with no grain showing through. Also if your using any of the transparent monokote colors use this technique to prevent color changes where the iron was used to try to bond the monokote to the balsa. If your concerned about the monokote sticking if your just ironing it down around the edges brush on some Balsarite film formula for extra adhesion.

Walt

[onethermal]

No need to iron it down over the entire surface of the wing or fuse. This will give you a nice smooth surface with no grain showing through.

Walt

yes it is true you will have a smooth look but you will loose the tinsel strength the covering was designed for some people think that covering is just that a covering and they forget there is another side to the covering.

One thing I have done to give it a smooth look is as you use a heat gun keep rubbing the hot covering down with a cotton glove or a soft clean rag that way you can get some of the glue on the backing to bind to the surface you are covering.

Clarence

[MatthewG]

yes it is true you will have a smooth look but you will loose the tinsel strength the covering was designed for some people think that covering is just that a covering and they forget there is another side to the covering.

One thing I have done to give it a smooth look is as you use a heat gun keep rubbing the hot covering down with a cotton glove or a soft clean rag that way you can get some of the glue on the backing to bind to the surface you are covering.

Clarence

Yep, what Clarence said. Although I don't know if I have noticed any affect in the strength difference. No doubt there is a difference I just haven't had it cause a problem. My building buddy made the same type of comments though so the covering gets rubbed.

[John Cole]

I've got a bit of information on a technique that sounds great, but I've not actually used it yet. I don't like to give advice about things I don't know about, but this idea seems to have merit.. ;)
In one of my old Multiplex kits, they address this very issue. It is similar to what Clarence suggests, but instead of a cotton glove, a piece of cotton material is stretched over a smooth wooden block. The wing must be sanded really smooth, and ALL traces of dust removed. The film is attached as described above, around the perimeter, then heated in the middle with a gun. followed closely with the block. This bonds the film to the high points of the wood, and the finish is supposed to rival painted glass. Wow, doesn't that sound fantastic??

If I ever get off these horrid hours at work, and fight my way to my messy work table, find my neglected Alpina, sand and prep the wings, etc.. etc... I will eventually use this very technique, and will report the results.

John

[Andy W]

.. but you will loose the tinsel strength ..

this just had me tickled! :D Imagining sailplanes dropping plenty of chaff for enemy avoidance..
..a

[mtnmnstr]

Done right...All good & tight. when it blows out:eek:, it makes a loud bang!:eek:

[schrederman]

Over open structure, you need the Monokote stuck firmly to the ribs and edges of the bay. That will give you more torsional rigidity as the Monokote will stretch less over a couple of inches than over the entire wing. I just use the sock and let it go at that. My sanding techniques usually keep me from having any grain problems. I use a large cotton ball and heat gun over sheeted surfaces. I think differing humidity plays some role in raising the grain. If you're in a humid area, there may be not much you can do to avoid the grain swelling the wood at different rates, and making it show through. Where I live, now, that's not a problem. While I was in Houston, I noticed that after a month or two, the covering job wasn not as smooth as it was when I did it...

Not much consolation, I know, but natural products obey Mother Nature... Oddly, humidity seems to affect Monokote and other coverings, too. While in Houston, I was constantly reshrinking it. Out here in New Mexico, it's rare to have a wrinkle, anywhere, except on my face... :eek:

Jack

[atmosteve]

Something to help adhesion is to paint a very thinned coat of goop on the areas you want your film to really stick. Monokote loves adhering to goop, and you can't see once covered.
400 grade paper followed by a light sanding with 800 grade gets balsa smooth as, remove any remaining dust with a good clean tack rag.

[dwells]

Thanks guys, it sounds pretty much like "let the ahesive do the work" and try not to use too much pressure. I'll use all this info...thanks again!

Don

[GDbot]

Goop? Very thinned? I don't understand. More detail please.

[atmosteve]

Marine goop, thinned with either toulene or xylene to the consistency of warmed honey. Its soaks into the grain of the balsa, not unlike acetone thinned dope. It has very resilient rubber like qualities. It creates a better adhesion on the covering surface.

People have been using it for yonks on foamies, particularly the slope boys whose aircraft tend to take a pounding. Thats where I first got to know and use it, on some of my slopers.

GD type 'goop' into the search function of the slope forum, and all will be revealed. :)

Cheers,
Steve.

[GDbot]

Thanks, atmo

[bobthenuke]

I tried the Multiplex technique that John mentioned on a Multiplex Flamingo without much success; the grain is quite evident, at least using Ultracote. It must be known however that I've never been real handy with plastic films, although the technique described in the assembly manual is not representitive of rocket science. I used a tack rag, fluffy real diaper with a wood block, Monocote Glove (remember them?), etc. Still no joy.

It should also be mentioned that I have a couple of Multiplex sailplanes that came with factory plastic film covering and even with the relatively tight grained obechi under it, the grain shows through like gang busters. So.....from 5 feet away they look great and maybe the "grain" works with the boundry layer to help with L/D. Or not. ;)

-bob

[Lost_Dawg]

I went to a covering presentation put on by a local club. The guy doing the demo has built 100's of models for other people for $. This is what I learned: Take some blasa scraps. Sand one to 220 grit. Sand another to 400 grit. Then try one to 600 grit. Cover each test piece adhering the covering to the wood 100%. Take each finished test piece and hold it in the light at the angle that will show the surface best and you will see a big difference in each piece. The man doing the presentation sands his wood to 1200 grit! I went home and tried his method but stopped at 600 grit. I'm here to say it works. My wife had no problem seeing the difference between samples when I showed them to her. Sanding like this may sound onerous but the fact is that once you get past 220 grit it only takes a very, very few swipes of the paper with each of the next finer grit to get where you want to go.

[atmosteve]

I went to a covering presentation put on by a local club. The guy doing the demo has built 100's of models for other people for $. This is what I learned: Take some blasa scraps. Sand one to 220 grit. Sand another to 400 grit. Then try one to 600 grit. Cover each test piece adhering the covering to the wood 100%. Take each finished test piece and hold it in the light at the angle that will show the surface best and you will see a big difference in each piece. The man doing the presentation sands his wood to 1200 grit! I went home and tried his method but stopped at 600 grit. I'm here to say it works. My wife had no problem seeing the difference between samples when I showed them to her. Sanding like this may sound onerous but the fact is that once you get past 220 grit it only takes a very, very few swipes of the paper with each of the next finer grit to get where you want to go.

I'll second that, I give up at around 800 grit. It makes a real difference.
Like Mr _Dawg says, it takes more time to roll a durrie than to get a fine finish with fine sandpaper :)
Besides, its therapuedic anyway.

[dwells]

Sounds like a plan. Steve, your finish looks great on your 3. did you use 800 on it?

[atmosteve]

I think I started forming with 180, then 320, then 400 then onto the 800 grit Don. Seemed to work nice for me anyway, the graduation of grit helped the even shaping of the LE and TE, and also where the D sheeting meets the ribs- very important for the look after covering.
The 800 grit helped me to get a bit of a knife edge on my TE, though that does nothing for its strength and resistance to knocks and dings. At least it will be slippery.
After brushing and blowing off the dust, my tack rag was simply a fresh cotton rag slightly dampened with water.
With the monokote; it may need a heat gun in curvy places to get remaining wrinkles out, the iron may not do it as easily as it does with profilm or solarfilm.

I'm yet to tackle the finishing of the fuse and tailplane though, currently I'm up to my neck in rebuiding my motorcycle engine between work days.

Hey Don, when you get the chance to work on your Oly III build, would love to see some pics.

[lincoln]

I seem to recall an expert scale builder recommending that you dope any areas you really want the Monokote to stick to. I wonder if Sig Sanding Sealer would be good. That stuff gets real smooth in a hurry. Sig Stix It is really good for adhering coverings, although I suspect Monokote over dope, done right, would be very hard to remove anyway. I prefer the Stix It to Balsarite. It doesn't creep as easily.

Dope is pretty easy to work with if you have ventilation. It dries very fast.

[dwells]

Steve, I'll start a build log on my Oly once I'm really into the swing of it. The fire has still got me moved out of the house and will take quite some time to get back to a normal life. I will post many pix as I go along. Thanks for all the info on your build, I can't wait to see yours in the air. Any ET for the maiden?

Don

[dwells]

Thanks lincoln, lots of recommendations for dope coating. I will probably coat all surfaces that will have film contact.

Don