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We often need to reinforce parts by laminating them with a layer of paper. There are two problems we may meet.
First one occurs on large parts: applied glue dries out before we manage to cover the whole area. This can be prevented either by some faster way of application (wide brush, squeegee etc., but avoid too thick layers), spray glue (more expensive) or double-side adhesive tape (some modelers like it, but I don't trust it - who knows what it does after five or ten years).
Second problem occurs when the glue cures: paper expands when damp and shrinks when dry, so laminated parts tend to roll. What to do? Drying under flat, porous heavy weight (for example between pages of a book) only helps partially: it prevents multi-axial deforming, but the part is still rolled. Lightly laminated parts can be flattened by usual methods (see below paragraph about cylinder shaping), but they may return to the deformed state after time, so it's safer to add formers or form it into non-deformable shapes (cylinders, cones, small faces etc.). Second (and very good) method which works best with thicker laminating cardstock is to glue second layer of paper on the other side. Strains cancel out each other and after drying in a book, you get a perfectly flat surface (if you're lucky). Last method is to apply glue in small, scattered dots instead of spreading it on the whole area. This is good for solar panels, signs, doors and similar applications where you need thin, flat thing with print on both sides and not much strength needed.
"Dry" glue sticks are quite good choice for laminating. They can be applied quickly (beware of premature drying) and they don't deform paper too much. Of course, they have disadvantages as well. First, they are weaker than liquid glues, so the parts can delaminate when too much force is used for folding. And second, not all sticks perform the same: some are too soft and leave chunks on the paper, some are too sticky and it's hard to apply them evenly. Try various brands and choose whatever you like best.
Sometimes we need perfectly flat surface, non-laminated tabs, sharp folds or to reinforce already assembled part. This technique can do it. Cut a piece of cardboard about one mm smaller on each side than the surface you want to reinforce. Test-fit the tile inside the assembled part and trim to fit. Apply only small droplets of glue, so the surface won't buckle.
If you need extra strength, fit the card pieces edge to edge and glue them together as well as to the parts. Connect with extra formers if needed.
Traditional way to roll paper is to pull it across an edge of a table. It works, but it needs some skill so as not to damage the part. If you use laser print or you're just too afraid (like me), you can use more foolproof technique: lay the part on a suitable soft surface (fingertip, palm, thigh etc.), outer side down. Take some cylindrical object (needle, nail, pencil...), press it on the part and roll it back and forth.
It works for cones too, just hold one end of the roller in place and roll only the other. Cone tips are trickier; you need something really conical and sharp, maybe roll the tip between your fingers when everything else fails.
If paper is too stiff and refuses to take shape, it can be persuaded by moisture (saliva is best - either lick the part directly or use a brush). Moist paper can take various sort of abuse, just be careful and don't damage printed textures. Laser print cracks, ink dissolves, offset can get scratched.
The main goal is to make the paper itself hold its final shape, even before being glued to something.
A solid layer of glue on a tab can terribly warp the whole part. The longer the tab and the thinner the paper, the worse.
This is a safer way. Take a lump of glue on the tip of some flat, pointy tool. Apply the glue in a separate, small drops spaced several millimetres apart, either along the fold line or somewhere in the middle of the tab, but never on the outer edge. The point is to isolate the glue by relatively large areas of dry paper which are enough to prevent the warping. The drops will spread to larger areas when the parts are pressed together. The resulting joint is strong enough.
This is a method to make your wings strong and undeformed even if you apply the glue too generously. Copy trailing edge of the bottom surface of the wing onto a cardboard. Add 6..15 mm inside, depending on the wing size, and cut out. Sand the trailing edge from the top side, until it is sharp. Glue the former on the bottom wing skin about 1 mm from the edge and let dry under a weight, then close the top skin. The former should prevent all sorts of warping.
It's very hard (sometimes impossible) to achieve really sharp and straight folds without scoring the fold line. If you have no other choice, at least use a vise or tweezers with straight edges to control the folding.
The paper can be either half-cut or just burnished. Cutting is better for thick materials or for extremely small parts, burnishing is safer (no danger of cutting through) and provides cleaner edges for thinner papers. I like to score with a strong blunt needle, a dull knife or depleted ballpoint pen should work as well.
Straight lines need a ruler, curves usually can be scored freehand (carefully), or with some curved template. Again, folded part should hold its final shape itself, so we don't need to overcome any forces or strains while gluing.
Take a wad of cotton wool, add some glue (normal white glue is OK) and roll between your fingers to make a homogenous mixture. This stuff can be easily shaped and stuck to places and is very hard and strong after drying out.
Useful for propeller hubs or similar round thingies on Polish models: fill the part with the gooey and close its petals around it. Also, you can sculpt complex round shapes which normally cannot be done in paper, or make internal formers where pieces of cardboard or wood wouldn't fit. A disadvantage is long drying time.
Nice transparent foil can be bought in copy centers. If you want thinner, ask for overhead projector slides; thicker is used in binding. Cheaper alternative is to reuse packaging material: sweets, toys, clothes, electronics, food... some boxes provide nice transparent bits of various shapes, sizes, thicknesses and quality.
Now, how to glue it. Water-based white glues (Elmer's etc.) don't grab on the foil, so we have two choices: either pour the glue around the edge to make a "lock", or insert the foil between two layers of paper, so it can't fall off even if the glue fails.
Or we can use different glue. In Europe, there is Chemoprén Transparent and UHU Alleskleber available and they hold foil very well (just be careful and avoid spills and smudges). I'm not sure about the rest of the world, choose whatever works for you. I don't recommend cyanoacrylate superglue - it's very strong, but its fumes usually fog the foil (which is bad, unless you're building a winter diorama). Double-sided sticky tape is quick and clean, but can't bear permanent load - if there are any strains left in the foil, it falls off after time. They are always some, unless you shape the foil perfectly before installation, which is of course recommended for all ways of gluing.
A foil can be folded almost like paper, but scoring doesn't help that much (you need to cut or press more). Cylinders and cones are difficult, because the foil doesn't want to stay rolled. Pulling along table edge can result in ugly white fold lines. Heat can deform and shrink the foil more than we want.
Really small windows don't need foil at all, white glue can do the trick. Take a drop of glue on a needle and apply it to the window frame (just the edge, not the surrounding area). Then take another drop, longer and thinner; its length must be at least the window width. Touch the needle to one side of the hole and pull it across it, like a wiper. The glue forms a "pane" similar to a bubble skin. Finally, wipe any excess glue and bubbles from the window. White glue is transparent when dry, so the result looks almost like glass. If it buckles after time, just breathe on it and it flattens again.
More robust way is to stick a transparent tape from behind and apply white glue on the sticky front side. This allows larger and stronger windows, but be careful - some tapes become yellowish after time and their glue can creep through paper.
Punch a hole for the axle only on the inner disc of the wheel. Sand end of the axle flat and dip it in glue. When you push the axle into the wheel, part of the glue stays on the inner disc and sticks the axle to it. Rest of the glue stays on the axle end and sticks it to the outer disc. Align the wheel to a perpendicular position before the glue sets. The joint is strong enough, but can be torn off if an emergency occurs. I like this system and use it wherever possible.
The little triangular tabs are the most tedious part of every wheel build, but luckily we can live without them. Cut the triangles off the tread ring. Make a strip of some suitable paper or card (depends on wheel size), exactly as wide as the tread, and of a length at least three times the tread's length (if you don't have such large paper, stack more strips). Assemble the tread ring. Roll the strip into small scroll, insert it inside the ring and let it unroll (it's usually not needed nor recommended to glue it to the tread). Take some pointy tool and help the unrolling until the strip is packed tightly along the circumference. Finally put a drop of glue under its end to prevent unrolling.
Now you have a nicely circular ring with edges wide enough to apply glue, so do it and stick side disks on. Note: white edges of the paper should be painted before this, because any possible gaps will be too narrow to allow the paint come in.
Paper models are usually pre-textured and all we need is to paint white edges of paper. But some models do need to be partially or fully painted, just like plastic kits. From the quality point of view, plastic modeling paints are the best - they cover well, come in a wide range of colours and don't warp paper too much. Their only disadvantages are the smell of solvents and a higher price. Fortunately, tempera and watercolours grab on paper very well, so we have an alternative. The following text is not a manual, it's more like a collection of beginner's experimental results with water-soluble paints.
New notes may be added later, when I fail in some spectacular way :-).