why do you need this zinc chromate ?
google search found this straight away
"Zinc chromate dust has the potential to cause short-term health effects such as skin irritation, rashes, ulcers and irritation of the eyes, nose and throat.
The National Occupational and Safety Commission (NOHSC) determined that there is sufficient evidence to establish a casual association between human exposure to these substances and the development of cancer.
Zinc chromate, as a dry film, is inert and presents no risk if it stays that way. It’s only when it is disturbed by sanding, grinding and welding activities that it presents a health risk."
Understandibly nearly everything causes cancer these days, but a good underbody sealant doesnt come with that type warning.
There are plenty of rust proofing products out there without such risks - my recommendation is the POR-15 range.
Just a heads up for you though, soda blasting will most likely cost you more, but there is less chance of panel demormation AND the residue it leaves protects the metal from rust until such time as you can properly seal the metal - ie the panel shop.