you can read a variety of opinions on any given subject, but a little common sense about the materials you are treating helps. IMO if it matters is dipping is the best way to achieve the goal of removing the rust,whether its between layers or inside frame rails. the key is to find the company that is auto related and informed chemical dipper. one that is Auto industry informed knows to look at seams that are compromised by rust and knows to separate the rusted panels enough to let the chemicals to their job this may sometimes add to additional cost and labor depending on severalty of the damage. you just have to find the right job shop. the very main reason to dip is to get all of the hidden rust without damaging the remaining metal that is not compromised.The the e-coat process can coat all surfaces hidden or not inside and out . the process is to remove all or as much of the comprised panels as possible and separate the seams then replace the panels that are visibly rusted then send to the dip and strip ask for a phosphate coating which can protect from surface rust for an extended amount of time so that you may examine for other damaged uncovered after dipping. repair as necessary send to e-coater and your ready for body work and paint.
soda blasting is corrosive as soda is crushed salt. that's just plain wrong in my book. sand is also corrosive and compromise's the finish of the metal and opens pours of metal . sand blasting is meant for heavy metals not sheet metal. ive used all methods available but dipping has the best results if done properly. soda or sand blasting does NOT get between the the panels. It depends if its about money or preservation , but quality wise the dipping and e-coating is the best option available and value for your buck .