A perfect paint demarcation between tire and wheel hub is one of modeling’s more ticklish tasks. Aftermarket masks can be bought for some of the more popular kits in larger scales; but for a small investment in tools and materials hobbyists can make their own masks for any size wheel, spinner, nosecone, or any part requiring a sharp circular color separation line.
The required tool is a set of hollow gasket punches, available for about $10 from most auto supply stores, hardware concerns, or mail order firms. Select a size range appropriate to the scale in which you will be working, and keep in mind that sets with more punches, though more expensive, will make it easier to find the correct diameter. You will need a smooth cutting board with the right mix of give and resiliency. I found happiness with a polyethylene-type sheet that entered life as a divider in a loose-leaf office binder. Thick styrene card stock would do; or you might talk Mom out of an unused Tupperware top.
Other implements are likely already on hand: a light hammer, hobby knife with #11 blades, forceps, toothpicks, and of course airbrush. Disks can be punched out of any type of masking tape, but the best results are from foil tape sold in the general hardware department as HVAC (Heating / Ventilating / Air Conditioning) repair or automotive section as body patching. This material cuts cleanly, comes away without fuss from its backing, forms an impermeable paint seal, and best of all can be stretched slightly by burnishing to expand to a perfect fit. About $5 buys a roll sufficient for a single modeler’s lifetime or for sharing among a club of moderate membership.
Prior to painting prep the wheel to remove mold seams and irregularities. One way is to mount it through the axle hole to a toothpick mandrel, chuck it into a rotary tool, and run at very low speed against a piece of soapy wet 400 grit abrasive paper. Paint the hub, then select a punch that matches the hub diameter. If an exact fit cannot be found choose a slightly smaller size; the disk can be enlarged by burnishing. Cut out the mask using a twisting motion while tapping the punch with the hammer. If necessary, the cutting edge can be hand honed inside and out with 400 grit paper, or with a fine polishing stone in the Dremel tool. With the proper light touch the tool will cut through just the foil; but if it goes all the way through the disk can be poked out of the tool and separated from its backing with the tip of a sharp blade.
Center the disk on the hub and use a toothpick to press down tightly around the rim of the hub. If the disk is too small, stretch it by burnishing outward from the center. If it still will not fit, or if it is slightly off center, additional sections of disk can be overlapped and burnished down to form a perfect circle. Mask both sides of the hub, then re-mount on the mandrel and spray paint the tire color. Brushing will also work, though it may leave a heavy buildup. When dry, pry up the foil with a blade tip, being careful not to scratch the underlying paint.
A slight problem arises when the hub must be painted with a NMF (Natural Metal Finish) that cannot survive masking with sticky stuff. In this case the process is reversed. First spray the entire wheel the color of the tire. Punch out an appropriately sized disk, but discard that, and instead use the surrounding “donut” to mask the tire. Burnish it tightly into the crevice where rubber meets rim, and if the hole is too small run a sharp #11 blade around the circumference to cut away the excess. When both sides are covered shoot silver paint through the donut hole. Let dry and remove the mask.
Whichever sequence is used, the result will be a sharp, perfectly circular line between tire and hub. Any “oopses” can be corrected by a steady hand wielding a ten ought brush or paint tipped toothpick.
Donut masking also facilitates contrasting coloration on nosecones, bomb and tank tips, propeller spinners, or any surface to which tape will not conform. For compound curves such as these, punch the mask from the backside so the resulting flange matches the part’s taper. When removing, avoid pulling the mask off, which can smudge the line. Instead carefully slit the ring at several points and pull the sections away sideways.
This technique will add realism to any subject, and leave fellow modelers wondering how you manage to paint with such a steady hand.