You might try soaking the frame in a molasses bath. (1 part molasses and 8 parts water). This solution is used to remove rust and I have used it very successfully. Usually, all bolts are easily removed after soaking for 2 weeks. Frame will be rust free also! Pressure wash to remove the molasses residue, and use some After Blast from Eastwood to keep the cleaned metal from flash rusting. GOOD LUCK!