Now please let me start by saying that KERRY's method (mentioned above) is BY FAR the more "correct way" to keep it straight while drying, but...… One of the guys that used to belong to our local hobby club used to heat the warped frame rails over the stove top, cool with water, glue together, then placed a brick on top to keep it straight while drying...... I can't be the one to criticize him..... his old stuff always seemed to have a straight frame!!!!
DJ