If you have a backsaw with a wavy blade,it is because the blade has slipped in the back. Clamp the leading edge of the blade in a vise,and tap the front edge of the saw's back so as to stretch the blade,and it will be jerked straight. THIS DOES NOT INCLUDE KINKS where the blade actually has a crease in it. The correct way to straighten the cutting edge of a back saw whose blade has developed a smooth curve,is this: Clamp the tip of the saw's back in a vise with the blade upside down. Adjust a crescent wrench to fit the back. Sighting down the blade,twist the back vertically. That is,hold the crescent wrench vertical with the back clamped in its jaws,and move the far end of the wrench's handle sideways to twist the saw back as if you were making a twist drill out of it (hopefully not!). Most people think only of bending the back to straighten the cutting edge,but then you have a bent back.

To straighten a curved crosscut or rip saw: You must pour boiling hot water on the blade,and instantly bend the curve out of it. You can bend a cold blade all day long,and not get it straight-IF it is made of GOOD spring steel. Most commercial saws aren't made of good spring steel,and have fairly soft blades that you might bend cold. The antique Disston saws,and saws of other good makers were made of much better steel than now.

There are basically 3 grades of spring steel: 1070,which has .70% carbon; 1080,which has .80% carbon,and 1095,at .95% carbon. Only the 1095 spring steel is hard and durable to make a real good saw,but it costs more,so manufacturers don't use it because most people don't know any better these days. 1095 is about 52 rockwell hardness,much better than the lesser grades. This is why the old timers said a good saw could cut nails. I don't recommend this,of course,but this is why the good old saws are better. In the old days,when hand tools were the tools used,craftsmen knew better,and makers had to meet their requirements to sell tools.