The only way to straighten a cane section is by heating gently to soften the resin in the cane and gently applying pressure in the opposite plane. Hot air is usually recommended; I use steam as there's less risk of overheating or scorching. Never had any trouble with moisture causing damage, but then I straighten the cane before stripping it; if it's a dud I haven't wasted any time, and the varnish protects it from moisture and excess heat. If the varnish coat is iffy apply a thick coat of quick-drying varnish before steaming.
Once straight, leave to set for a few days before flexing; cane contains pectin (like jam) and the longer it's allowed to set, the harder it gets. If after this treatment you find the set returns in one direction but not the other, it's likely the rod has been overstrained at some time, causing degeneration of the power fibres on the outside of the curve. This is unfortunately irreparable. If the set seems to return whichever direction you flex it, the six segments may be delaminating, or the cane may be poor quality and unseasoned. The latter is destined only for the wastebin; the former is treatable by separating the segments and re-gluing, though it's questionable whether this is worthwhile, as it represents about the same amount of work as making a new section.
Hanging weights off the tip or fixing it between two rows of nails is plain daft.
Well I've had some good results hanging weights Fred, but not always it's true.
The last effort failed. The moment I flexed the rod the set returned and sadly it's a sideways set, propably from sitting with a centrepin in hand for many, many hours...which is just what I wanted the rod for .
The first rod I did like this was a greenheart tip with a near 90 degree set in it. Amazingly it worked. Others haven't
John Chapman says to use dry heat and didn't like my steam iron idea, but you're not alone in saying it can be got away with if there is still sound varnish to protect the cane from damp.