With proper care, spray cements can provide a lighter, more uniform adhesive for Ultra film than any brushed-on coating. Brushed-on adhesives are difficult to control and take much longer to apply. A light spray adhesive will add less weight. One of the best features of spray cement is that it allows repositioning of the film if a mistake is made while covering.
We recommend 3-Ms Super-77 or 75, and Grumbacher-548 with fine, transparent sprays. The Super 77 is a high-tack adhesive that now comes with an extra fan-spray nozzle ideal for indoor models. For the lightest covering, the Super 75 is best. It has a lower but more than adequate tack, and is formulated for temporary bonds. The air load on indoor models is so low that this adhesive is essentially permanent. Grumbacher-548 sprays uniformly, has moderate tack, but is heavier than Super-75. It has an orange tint that makes it easier to see.
When setting up to cover a model, it is very smart to spray the wing, stab and rudder frames outdoors. This is the safest and least messy way to do the job. The cement will stay tacky for a very long time, so rushing back to the building board isn’t a priority. If you must spray indoors, be sure the room is well ventilated. This means fresh-air circulation (i.e. open windows). Also, cover the floor where you will spray with lots of newspaper, at least three feet beyond each edge of the largest frame you will spray. Spraying in a large, open-top box will help contain the overspray.
To help see where you are spraying, try this: shine a flashlight horizontally across the spray zone a few inches above the floor. Turn out all other lights (or spray outdoors at night) when you spray. The aerosol droplets will reflect light from the flashlight so you can see them more easily. This is a good way to estimate how much you are applying, and where to spray has been Applied.
Use just one pass on a narrow wing, and no more than two on a wider wing (one each for the leading and trailing edges). For Super-75 and Grumbacher-548, hold the can 12 to 24 inches above the frame. For Super-77, use 24 to 36 inches. Spray about one foot per second. You will be surprised how little adhesive is required. Remember, less is lighter and weight is the enemy!
The very best covering method is to cover the wing flat and add dihedral later. If the sprayed frame is put over a traced outline on the board, it can be adjusted to eliminate skew and wiggly outlines. You may need to tack-glue it to the board if it doesn’t want to stay in place. There is no rush; the spray adhesive will stay tacky enough for several hours.
Ultra film can best be applied either from a light balsa or pine wood form approximately 2” larger than the model structures to be covered, or two people can hold the Mylar with a hand at each corner. A little practice will get smooth coverings without crushing curved ribs. If you make a mistake, carefully peel back the covering and rework it. Lightly press on the outside to secure the Ultra film. Then some builders cut or trim the excess with a pencil soldering iron or wood burning iron (a 23 to 47 watt iron with a thin chisel tip is good). Some advanced builders think that this is much easier than trying to use razor blades, as the heat seals the film edge and prevents rips that might readily propagate. After cutting off the excess film, turn the model structure over and press the ribs to secure the film .
FAI MODEL SUPPLY likes a slightly different approach as follows:
A. Make a wooden frame using two wooden ends 3” wider than the width (chord) of the wing.
B. Obtain two wooden dowels that are 4” longer than the length (span) of the wing.
C. Make holes in “A.” above ˝” in from each end of the wooden ends of the frame and glue the dowels into these ends to make a rectangle frame that is larger than the outline of the parts to be covered by the Mylar
D. Cut film that is larger than the new wooden frame just constructed and carefully wad the film up into a tight ball.
E. Lightly spray the new frame with contact adhesive.
F. Ca refully unwrap the Mylar ball and gently, via 4 hands, stretch out the film and attach the film to the new wooden frame. NOTE: do not stretch out the film so much that the hundreds of tiny wrinkles disappear. These wrinkles will give a slightly cloudy appearance and will slightly enhance your models performance.
G. Spray the model part to be Mylar covered.
H. Now place the sprayed model part upside down on the stretched Mylar
I. Gently press down on the model structures to insure that the surfaces are stuck to the Mylar.
J. Now carefully trim the Mylar with a new EXTRA KEEN RAZOR BLADE. By holding the blade at a 45 deg angle to the film and next to the part structure, the model part can be very neatly trimmed from the film
Dihedral in wings can be added at this stage. Cut the joints at a 45 deg. angle to increase the gluing surface. The film near the dihedral rib will loosen. To draw it tighter, wet a small brush in spray cement (from a spot sprayed on paper) and lightly coat the film on top of the rib. You may want to thin the cement with some toluene (Elmer’s contact cement solvent) to make brush easier. When the coat gets tacky, gently nudge the loose film against it with a flat (but dull) tool or balsa sliver, from below the wing. Be careful not to push too much film onto the cement strip or the dihedral rib will bow excessively. If this happens, pull the film apart and rework it. This technique requires some practice. But remember, loose film is not too detrimental to long flights.
A better solution might be to dip a small pointed paintbrush into some COKE or PEPSI. Carefully draw the brush across the loose Mylar joint. The loose film will be drawn together.
These covering suggestions should get you well on your way to indoor modeling without the frustrations encountered with paper.
Japanese tissue, though not recommended due to heat/cold, moist/dry atmospheric conditions, can be used by twice water shrinking and drying the tissue on a carpeted floor. On a flat, smooth, dry surface, iron the tissue just prior to covering the model parts.