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Builder's Corner
By Mark Kallio
| Over the last few issues, I have discussed building and
covering techniques. We’ve discussed how to build the model straight and
light, structural considerations that need to be addressed, trim patterns,
trim layouts and color visibility/selection. This month we’ll cover MonoKote
characteristics.
First, I should say that I only use Top Flite MonoKote as I have had good results using it and feel I'm very familiar with it's characteristics. Each specific brand of covering has it's own distinct quirks. By sticking (pun intended) to one type of covering material, you will become familiar with it and hopefully become better at covering your models with them. Point being, DON'T keep changing the type of covering you use. All major types of covering have their strong and weak points and are basically as good as the next. If you don't use MonoKote this article is probably not for you but it should be entertaining none the less. Just like I decide the trim pattern I'm going to use before I start building the model, I determine the colors I'm going to use as well. There are four types of MonoKote available: metallic, frosted or pearls, opaque, and fluorescent. Two types have certain characteristics that can be used to you're advantage. First, metallics have to be the best of all the types. They bond extremely well, go around corners like you wouldn't believe, have exceptional shrinking abilities, and best of all make seams almost disappear. They give the model character because the color you see changes as the reflecting light bounces off the model. I love metallics! O.K., so there is a drawback to using them. Trying to get a paint to match them perfectly is difficult but not impossible. But take my word for it, the search is worth it. Second, opaques (white, cream, pink, blue, red, etc.) are just about as good as metallics and probably give you the best all around workability. They bond, shrink and go around corners well. It's real easy to find matching paints for just about all the colors. One word of caution when deciding to use white as a primary covering color. White MonoKote, although sold and described as opaque, is really simi-translucent, and the seams (as well as dark spots on the balsa underneath) do show up vividly at a few feet away. If you like using white, which I do, plan for this during the construction phase of your model. Splice white on white seams in inconspicuous areas of the plane and be sure not to have dark marks on supporting structures beneath. On the last two types of MonoKote, I've never used frosted or pearls but I assume they are similar to the metallics. Finding a matching paint, well.......... I have enough problems with paint matching, thank you! Also, common sense tells me that these colors are simi-translucent, just like white MonoKote, so using them could cause some problems with splicing. Fluorescent MonoKote colors are the worst covering I've ever had the displeasure to work with. They don't want to bond as well to the wood. They are thicker and weigh more that any of the other types of MonoKote. However, they are very bright and visible. Will I ever use them again ? NO WAY!!!! Before we start talking about putting MonoKote on the model, it would be helpful to talk a little about the characteristics of MonoKote as they relate to covering. MonoKote shrinks not because we apply heat, but because of the cooling down of the covering after we heat it. So remember to give the covering a cooling down period in order to achieve a uniform shrinkage. Sure you can heat it up and shrink it without cooling off but this causes uneven shrinkage and usually will cause the covering to "release" (go slack) in another area. This applies to stretching the MonoKote as well, something I like to refer to as "stressing" . By pulling and yanking and stretching this stuff around, the covering gets "stressed". Pulling the MonoKote taught is one think, trying to get the stuff pulled so tight that it doesn't need heat to make it tight as a drum in another. If you stress it one way, it will go slack in another area even if all the sides around the opening are securely bonded to the structure. Never, ever "stress" the MonoKote, you'll live to regret it. MonoKote also has a "grain". That is, it shrinks more in one direction (spanwise) than it does in the other (lengthwise). This can be used to our advantage when cutting out the covering for a wing. The manufacturer must have realized this as well because your longest cut is generally longer that the width of the roll, making it almost impossible to get the grain turned in the wrong direction (But guess what? Nothing is impossible and I've........ well, you get the picture). The same applies to covering the fuselage. The grain of the MonoKote naturally falls into place as your longest cut is again, generally longer that the width of the roll. So here we are again in our workshop with our model ready for covering. right? Well. just to be on the safe side let's check the surface of the model for any nicks or dings that have not been filled and sanded. Let's also check all the joints to insure that they have been sanded smooth. Now it's time to blow off all the sawdust and get this sucker ready to cover. I have found that by attaching the shop vacuum hose on the exhaust (blower) end and using a crack-and-crevice cleaner on the end of the hose, I can blow off just about all the dust anywhere inside or outside of the model. Be sure to do this outside the workshop area as all that dust will take days to settle (two guesses how I found that one out!). Before bringing the model back into the workshop this is an ideal time to do a final clean up around the shop. Remember, when covering, dust is the enemy ! ! ! ! ! ! The final touch is a complete wipedown, inside and outside, with a good tack cloth just before you bring the model back into the workshop. Keep the tack cloth handy at all times during covering (you'll find out why later if you haven't guessed by now). You should now have your workshop clean, model ready to cover and your tools all laid out. Below is a list of what you need as a minimum: Heat Gun
Well I know your saying "This sounds like a heck of allot of stuff. I know you don't need all that crap to just cover a model". Please let me point out that most film manufacturers' instructions show the Ieast expensive techniques. Don't be mislead into thinking their methods can achieve award winning results. At best, finishes using those suggestions will be presentable. I'm sure if you have made it this far into this article, that presentable ain't good enough, it surely isn't for me. The finish most modelers want is one which sets the model apart from the rest, a true work of a craftsmanship. The main covering iron we'll be using for most applications will be the one set at 300 degrees. With that setting and the hot sock on, the surface temperature is about 250-275. Fairly low temperature for using MonoKote but let me explain why this works so well. I have experimented quite a lot with temperature settings and techniques to seal covering to balsa and the best way I have found to date is to use "Balsa Right". (Just for clarification, when I refer to "Balsa Right", I am referring to the type for film unless otherwise noted.) Balsa Right was designed to work best with low-temp films. Too high of a temperature and the Balsa Right starts to blister or " boils". However, the superior bond it produces allows you to do things that can not be done any other way. For example, on most my planes I cover the bottom of the wing in alternating colors. With the trim pattern on top of the wing running in the opposite direction (lengthwise) it's extremely easy to distinguish the orientation of the plane during spins, snap rolls, etc. With a maximum bonding area on the cap strips being 1/4 to 3/8 inches and the fact that you have to shrink the MonoKote in the openings between ribs, MonoKote bonding strength to the cap strips is essential in order to make this work. Also, have you ever tried to cover 1/64 or 1/32 plywood ? The MonoKote does not want to bond to it. When you finally get it to bond, just one day in the sun will undo most of that. Another thing to point out is that the lower heat setting distorts the MonoKote to a lesser degree around the area being attached to the structure. This makes the covering much easier to work with. Lastly, the better the bond to wood, the less likely the chance that you'll get bubbles under the covering. Balsa Right is great stuff if you remember that it doesn't like high temperature setting when applying MonoKote. I've never had any blistering problems because of direct sunlight on the model. These temperatures rarely get above 150 degrees, well below the temperature required to reactivate the glue in the covering or in the Balsa Right. First, let's go over some basic covering techniques. Take a flat piece of balsa, say 6"x 12", sand it smooth, clean it (don't forget about that tack rag), then use you acid brush to apply Balsa Right around the edges and one side. The Balsa Right will dry in about 5 minutes. Place your 300 degree "cold" iron on one end of the covering along the back edge of the wood and let it rest there for 3 to 4 seconds. Apply only light pressure to the iron, let the heat do the work. Now slowly move the iron across the length of the wood, pausing at intervals equal to the area of the iron shoe for about 3 to 4 seconds each. At this point you should have a 2 to 3 inch strip attached to the wood along it's entire length. Now working from one end (it doesn't matter which), you attach the remainder by moving the iron across the surface in a semi-circular pattern pushing towards the unattached portion of the MonoKote. What your doing here is pushing the air out from underneath the MonoKote as well as eliminating the chance for wrinkles to form before you attach it to the wood. Now if during this process any bubbles start to form underneath the film, this may be due to too much heat. Just move the iron a little faster or reduce the heat setting. Also, don't keep going over the already attached MonoKote with the iron. Overdoing this tends to form bubbles as well. Done correctly, you'll end up with a glass smooth finish THE FIRST TIME. With a little practice, this all becomes second nature. Don't use this method on your model until your practice pieces come out good, as with this method; You'll only get one shot at it. If however you do mess up, remove the entire piece of MonoKote, re-sand the model lightly and try again. I've found that Balsa Right will last through at least one "re-cover job" prior to having to re-apply it. In fact, the second time around it seems to work even better, with hardly a bubble. In the event that you get it down right the first time, but have a few bubbles none the less, here's the ticket laddies. Turn the cold iron down 25 degrees, wait a minute for it to cool down. Then poke the offending bubble with a NEW Xacto knife in a few places. Get the now cooled down-cold iron and go over the area using a very slight pressure for 5 to 8 seconds. If you did this right, then you'll never notice where the bubble was. On the other hand, if you do this wrong you'll immediately know because you'll usually have more bubbles under the covering than you did before you tried to fix things (remember; going over the MonoKote after it has been attached will tend to form bubbles... unless the iron temperature is reduced). If this happens, turn the iron down even further and try again. Most of the time it's almost just as advantageous to start all over again. To form the MonoKote around the edges, just use your trim iron (set on "high") and run it along the edges at about 30 degrees from the covered face of the wood. You should do this without stopping the iron along the edge as this tends to concentrate heat and will cause the film to "boil" along the edge. Now, set the trim iron aside and get your cold iron. Run this iron along the edge in a similar fashion as we did with the trim iron. After the first pass, start limiting the area you working on to a maximum of 6". Gradually increase the angle of the iron going back and forth along the edge until the MonoKote is attached to the wood. Let's say that this edge was rounded, like the leading edge on a vertical stab. After covering the flat surface, cut the excess MonoKote from around the edges and use the 'hot' trim iron to seal things up. One tip I use quite frequently, is to cut a stripe of MonoKote about 1/8" to 3/16" wide and go around the edges of the fins, stab & rudder. Attach this with the "hot" trim iron as well. You'll be amazed just how easy MonoKote goes around these edges and corners when using a narrow piece of MonoKote. These procedures probably will apply during 90% of the MonoKote job. I know it sounds complicated, but it really isn't hard. In fact, it's extremely easy after you get the hang of it. So practice it awhile, then cover your model. By the time your done you'll probably be an expert at it. The next few paragraphs will deal with all the problem areas encountered during a typical MonoKote job. Although they include less than 10% of the total job, they generally take up the most amount of time. So read through them a few times before you get started. Be prepared; You are now entering the "Dark Side" of the MonoKote world. When applying MonoKote to the center section of the wing, you always have to cover over fiberglass tape. If you mix up a batch of Balsa Right using 50% of the kind for film and 50% of the kind used for fabric, I'll personally guarantee that this stuff will bond MonoKote to polyester resins, epoxy resins, or just about anything else for that matter (epoxy surfaces should be cleaned with acetone prior to the application of the 50/50 mixture). I usually apply the fiberglass tape with CyA and use this 50/50 mixture on it and this seems to work best. I also never try to taper the edges of the cloth to the wing using filler to make up for the thickness. I find this to be a real pain in the !@#$, not worth the time or effort. Fiberglass cloth ain't too thick, and besides, it's barely noticeable. There is one exception to every rule and this holds true when using Balsa Right. Don't use Balsa Right on compound curves. The adhesive is so aggressive that it doesn't allow the MonoKote to stretch/shrink before it bonds it to the structure. If you do accidently get Balsa Right on a part of the structure that you don't want the MonoKote to aggressively bond to, simply let it dry to the touch and wipe the area with, acetone. But beware that acetone will weaken glue joints and is incompatible with most types of fillers. Now I know some of you are asking "If Balsa Right doesn't like high temperatures, why do you want to have two irons on high temp settings?" The best way to answer this is to say these irons are "specialty irons". That is, they are used for one purpose only. The larger hot covering iron would be used to shrink an area of MonoKote prior to bonding it to the structure. This typically happens when you are covering the open structure on a built-up wing. The method I use to cover open structures is to tack the covering in one corner. then pull the covering taught and tack it down in the opposite corner, then pull the covering taught again and tack down the other two corners. Then, divide the open areas in half and tack down the covering. Keep repeating this process until the distances between the tacks is about 6 inches. Now work from one tack to the other using the "hot" iron. Don't be heavy handed and bond the MonoKote to the structure at this time. What your trying to do is to shrink the covering between the tacks before you bond it to the structure. A shield made of the MonoKote backing works well when placed between the covering and the wood insure that bonding doesn't take place until you want it to. Now after you have shrunk the area between tacks, go ahead and use the "cold" iron to bond it to the structure. Repeat this process until the entire perimeter is sealed. With practice, you can use the "cold" iron to do the majority of the work and eliminate the need for the shield. The trick here is to not put any pressure on the film at all, glide the iron over the top of the film to shrink it. Then when the wrinkles and puckers are gone, attach it to the wood. A final pass over the edges with the "hot" iron will insure good bonding to the structure. But work fast, too much concentrated heat will bubble the MonoKote. After that, use the heat gun to shrink the MonoKote inside of the open structure. Never. ever blow hot air directly on the attached sides of the MonoKote. I always use stiff cardboard to protect this area from the heat. If the bay openings are of uniform size, a good idea is to cut an opening in the cardboard 1/4 inch less than the open area and about 12 inches long. This allows you to shrink the covering more uniformly over the entire area. If there are not going to be any splices over a cap strip, do not use Balsa Right on them. The covering looks much better when not attached to the cap strips. If the covering does attach itself to the cap strips during the shrinking process, use a piece of scotch tape to pull the covering off of the cap strip or substructure then re-heat the area. Now let's talk about compound corners like wing tips. There is no significant aerodynamic advantage in having wing tips shaped like this as far as it applies to model aircraft. Personally, I always square off the wing tips. It makes covering 10 times easier. Sometimes, compound curves cannot be avoided. In order to cover these areas you must stretch the MonoKote while applying heat. It helps to concentrate the pulling and stretching to a small area at a time. This is where the small trim iron set on high temp setting helps out tremendously. While covering these areas, you might (and probably will) come across small wrinkles in the covering as you attach it to the structure. This is caused by too much film with too little surface on which to attach it. In order to eliminate this, pull the covering away from the balsa and apply heat with the "hot" trim iron while holding the covering slightly away from the surface. By shrinking the covering before ironing it down, the wrinkles should disappear. Now you can see why you don't want to use Balsa Right in these areas. If you still have wrinkles on the extreme tips of the wings, lift up the MonoKote until it's barely attached to the structure, pull out on the covering while at the same time, attaching it. If you get to a spot where you just can't get the darned wrinkle out, just slice the ridge lengthwise with a sharp Xacto blade and seal each side of the cut separately. Be careful not to cut into the open area as then, well..... it's time to start over. Now it's easy to see why I just square off the wing tips. It's way too easy to get yourself into this type of trouble! |
