The key to heat stretching sprue is the proper heating of the piece of sprue you are using. I hold my piece, 1.5-2" long over a lit tea candle and constantly roll it between my fingers. If it gets dark gray and sooty, you are too close. As the sprue warms up, put a little pressure to bend on it as you continue to rotate the piece over the flame. When you can get a 15 or 20 degree bend and can rotate smoothly while bent like there is a universal joint in the plastic, then you are ready to pull. Pull evenly and steadily. You can get the full span of your arms, about 5 ft, if you do it right, but 3' or so is easier to work with. Best to then drop one end and let it cool vertically. Look at it carefully and you will see it is thick and tapered at the ends and way too thin in the middle. Cut these unusable sections off and get rid of them. You should be left with a couple of pieces several inches long that are pretty close to even diameter and usable for rigging.
That is a lot of waste, but the sprue was waste to begin with. Only use straight lengths of sprue with no branching or nubs along their length. Different thicknesses require different heating time to heat all the way through. Sprue stretched for rigging needs to be heated all the way through. Sprue stretched for thicker pieces such as landing gear struts or radio masts is not heated all the way through and is stretched much shorter, about a foot.
Sprue for rigging can be painted before use by putting a drop or two of paint in an up turned cap and dipping a cotton bud in the paint and using it to press the sprue into the paint and then gently draw the line under the bud to coat with paint. Allow it to hang vertically until the paint is completely dry. Or, you can paint it once it is installed and then figure how to remove the little droplets of paint that get all over your wing. (Hint, make sure whatever coats your wing is impervious to whatever thinner the sprue paint requires. Then it cleans up easily)
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