G'day ron
I have never attempted to clean a scrim or for that matter a Cyclorama.
I think VickiFrank would be the scrim & cyc guru on this question.
I have known of others who tried & they ended up getting a new one.
These were cotton scrims & for the most part they rotted away in the washing machine process.
Normally with scrims I ensure they have a canvas or cotton bag to be packed away into. or bag the tail of the scrim, then hiost on the string pull of the duffle bag & tie it off the deck, while not in use.
In the most part depending how big it is, the only way I have heard of, with badly marked scrims. Is to soak it in 'Nappysan' [Diapersan - in your money]. Drop it in a trough sprinkle a cap full nappysan in cold warm water & leave to soak over night. Wringe it out then drape it over a washing line, or either fly it out on a batten or arrange a series of lines & spread the scrim out. Much the same as they did with dyed cloths, in the 1800's. By laying the cloths out in the sun. Supported on 'tenterhooks'. Which was about 3 lines tied off to poles, like rows of camp tents - hence the name 'tenterhooks'.
You will find it will dry fairly quickly, even dripping on to the stage.
the nappysan detergent is good, as it designed to be soft & gentle on the fabric, but has an active detergent. Releasing all those secret active dirt busting enzimes, making it as soft as to touch as a babies bum. [Or is that KFC take away chook?]
Lets face it if it gets the crap out that turns up in babies nappies, it should work on a scrim [it also won't hurt the colours either!]
However if your scrim is not preshrunk, it will be after washing it, that is if it's raw cotton.
If you don't want to fly it wet, wait untill it's damp & almost at the dry stage, rehang it & get tke tail batten [pipe] in it's pocket. allowing it to dry in situ. If wrinkles form spray it with water, from a cheap plastic hand held sprayer, while it is hanging.

Yur Bewdy Vicki - washing is rather haphazard at best - never thought of using the old 'hoover or 'sadie!
Must remember that - G'donya!
It would work well on on a cyc that some clowns touched or ran thier hand along, dislodging all the dust, Which looks like a streak under wash lighting.
I should have thought about the fire retardant? I'm a complete wally!
Although no one here bothers with fire proofing anything on stage, be it pro or ameatre.
The fire prevention act is a bit of a farce, for entertaiment venues. Because it is administered by the Local Council health depaertments & not the Fire Authorities.
Any breaches are handled by our full Councils to decide to prosecute or not. So local politics are played out, by all the Council flunkies, bless thier cotton doodats.

Being an Electrical Fitter & endorsed installer [mechanic] by trade.
 At one stage i was one of only two Licence Electricians employed in theatre through out the state.
Yet we had over 20 pro venues, who used techies, that had picked up the knowledge thru experiance, who were not licenced or qualified.    Although the Electical Workers Board had introduced Limited & Restricted licence schemes, for those in allied trades & associated industries.
Such as Refigeration, plumers & the entertaiment industry. However they had to complete a short term technical college course & be under the supervison of a licenced sparky for upto 12 months.
The entertainment techies still complain bitterly that they have to be licenced & most never bother anyway. The only licences they tend to have is for Pyro & fire arms handling supervision, only in theatre.
I atempted for years to have a form of modified restricted endorsement licence, specificaly for community theatre techies. As it was back in the 70's which was jnown as a 'Pug & Cord' Licence. Which was easily enough obtained, the only prerequisit was a colour blind test & a practical set test of changing out plugs & cords, along with questions on Safety & demonstrate electricution resusitation skills.
I lobied the Department & State government Minister to no avail.  The answer was always no person can  gain an sort of electrical licence, unless they are employed full time in that field.So unless community theatre employed thier techies, they could not aply for a restricted, limited or modified electrical licence.
Needless to say licenced electrical techies in community theatre are scaser than chooks teeth.
However they did make a concession with the  performing  arts colleges, where by the could aply as part of the course, for a licence, If the school employed a full time licenced qualified electrician, on staff as a lecturer. therefore the student would comply with the act, by having more than the minimum 12 months supervision of a licenced sparky.
But to me that is where the government fopps fall down, after working with a lot of graduates, who are licenced, that have no idea what it's all about & even to the piont of an understanding of Ohms law as applied to the SAA Wiring rules & regulations, even thier colour perception is rather suspect at times.
In the mean time i doubt there are a handful of comeatre techies & well meaning volunteers, fair any better.
However when I approached our state community theatre association on the problem & for them as a body to support  this endeavour. They didn't want to know anything about it. Mainly they were worried that they might loose those not prepared to be licenced, even in a modified test arrangement.because it would create a selected pool of techies & might cost the group money, restricting the available volunteers & they might ask to paid or expect a priverledged position within the group.
so I was shot down in flames & left out on a limb.
Are well I'm too old to worry about it all thier moronic fiobles now!
By the way this was a regular cyc problem in venues, keeping the paws off the cyc, is not a problem with me at the moment, as the group are theatreless vagabonds.