You got a lot of great and professional advice for free!
I work for the Fed Gov't and we go those hygiene guys around all the time.
God made those lungs for air; not a bunch of crap floating around in the air.
Get a RESPIRATOR; if the package does not say "RESPIRATOR" it isn't one.
The filters are done when you taste what you are trying to no breathe or 8 hours. There is no APPROVED long term storage of filters once they are removed from the original packaging. We are required to have our respirator in a food quality zip lock bag whenever it's not on our face. When using your respirator always wipe it out with alcohol or other approved sanitizer prior to putting it on. You'll notice fewer colds.
Yes it's expensive so plan your sanding and painting so as to use your filter time wisely. Do all your filler work on as many panels as possible, THEN sand everything; then do any additional filling required on all the work, then sand again.


1999 GMC Yukon
1957 GMC 100 Panel