C: "Could you possibly elaborate on some of the stories we are hearing about the problems with soda blasting?"
R: Happy to elaborate. When I was a kid in the 60's my father gave be a little printed sign that said, "When all else fails... read the instructions - TWICE!" I still have this at my workbench today and made a copy for my son.
Problems occur after blasting when the base surface (Metal or Fiberglass) is not properly cleaned and prepped for painting. Notice, I didn't say Soda or Abrasive, the reason is that IF you do not properly CLEAN and PREP surfaces after abrasive blasting, soda blasting or chemical stripping you will have problems when you paint -- guaranteed. Cut corners to save time... you'll have problems. Don't take time to clean thoroughly... you'll have problems.
C: "There seems to be a ton of varied opinions about the suitability of soda blasting for prepping old sheet metal."
R: Old sheet metal or new sheet metal the problem boils down to 3 words ... RUST, TOOTH, and PREP.
1. RUST: The age old problem is how you deal with Rust. Bicarbonate Soda, without abrasive additives, will not remove rust. The deeper the rust the more aggressive one needs to be with the Blast Media. Example, a 100/170 grit Glass is finer and less aggressive than a coarse 10/40 grit Glass. Bicarbonate Soda was the product used to safely clean the Statue of Liberty (inside and out) in the mid-80's, whose metal is older than most vehicles.
2. TOOTH: The complaint I hear the most is, "....Soda Blasting leaves no tooth on the metal for the paint to adhere to!" Soda Blasting de-paints metal back to the original finish -- the original "factory" finish. If the OEM was able to paint on the original metal (without tooth) it stands to reason anyone who properly "Cleans" and "Preps" the metal or fiberglass should have no problem painting. All Bicarbonate Soda does is removes soils and paints back to the original surface finish. If you want to assure a tooth, use a fine nylon kitchen pot cleaning pad (my wife makes me use one after she cooks) during surface cleaning.
3. PREP: It doesn't matter if it's abrasive, soda or chemical if you don't clean and prep properly you'll end up a very "unhappy camper" and have to do the job over.
C: "For every person who swears by it, I read horror stories from another about the problems they had when it came to painting metal that had been soda blasted. Is there some cut and dried method of cleaning the sheet metal after soda blasting that will prevent any problems with primer and paint?"
R: Soda Blasting leaves a fine residue of Bicarbonate Soda on a metal surface -- which is an advantage as it prevents "flash rust" between blasting and painting. The step people miss (which creates all the problems) is cleaning and prep.
Here are the steps needed to clean sheet metal after soda blasting:
1. Take an air gun and thoroughly blow-out all seams and crevices. (repeat this several times)
2. 1+1=2: Take (1) clean pail of hot water + (1) polyester microfiber towel wrung well to being damp-not soaked = Application of cloth to metal and wipe. Rinse and repeat often. Replace hot water as it becomes dirty.
Obviously, I being a little cute here but what I'm getting at is that cleaning isn't difficult. The key is clean hot water (no soap necessary) with a damp (not soaking wet) microfiber cloth (no lint) and wipe surface areas several times to assure all particulate film is removed.
3. Now, prep as you would for any paint job.
With Abrasives or Chemical Strippers you have to follow the same steps, except with Abrasives and Chemical Strippers you spend more time and more money on chemicals to remove and neutralize the chemicals. Plus, Bicarbonate Soda is water soluble and is not an environmental hazard; you won't have to worry about chemical disposal. Bicarbonate Soda is FDA-approved and EPA-approved.