Ruined many a shop vac with fireplace ash but I'll be darned that I did it hundreds upon hundreds of times with a garbage picked Fantom Thunder and it still works. Only issue is if a large coal jammed the hose.
I wouldn't use the Rainbow for it, and personally I wouldn't recommend many machines for it. There used to be an ash vac attachment to do specifically this, dunno if it is still made.
I don't think rubbing alcohol would be a good idea, while it creates sheeting action it disrupts surface tension of water, which would probably hurt the dirt trapping of water. Jet Dry is more of a water softener. Oil would have its own density and would float above the water (or be stirred in in churning air as you can tell when you add an oil based fragrance to this or their aroma therapy machine)
Now if you think its easy to kill these machines, it may rust the motor, may clog the HEPA, but killing the machine is no easy task. I went to someone's house that used one to suck up drywall dust and did so without water. Machine was a mess but still worked, was a single speed E Series.
There are reasons 30 year old Rainbows are usually still working, just like 30 year old Kirbys, they're built well enough to tolerate quite a bit of abuse. You're going to have to do a lot more to kill it than a $50 department store model.
Really my advice would be don't vacuum up drywall dust and fireplace ash & don't store it on the basin if the basin is not 100% dry. If I vacuum every day with it I put the basin in the basement where I rinse it. If I don't use it for a few days I put it back on when it is thoroughly dry.
Other people forget to clean the seperator, usually you don't run into problems with this unless you vacuum up too much stuff without dumping the water.
If you use a fragrance in the water at least you can eliminate the need for carpet fresh. I'd never use that stuff the grit alone can't possibly be good for the carpet, not many machines could pull it up sufficiently.
Peronsally I bought my E Series from a thrift store for $20 in like new condition. I still take care of it as if I paid a lot, knew the machine going in before I got involved with selling them (and already had a D4, also second hand). I'd get many times what I paid in trade for a new one. Despite all that my HEPA filter is in really good shape. They won't pass much dirt to the HEPA if cared for and used properly and mine still has the old brush motor.
