Fire 'em in the washing machine on a 90 degree wash then tumble dry 'em to start.
Try and get a batch done so they're "broken in" a bit, not to sure how many you've got but we get through about 10 a day per man. If you've got enough scrims use one for the windows and one for wiping sills this will shorten the time it takes for you to "break them in" as you're doubling up your scrim usage. Put all scrims through the washing machine every night. By the time you've got 6 weeks work done they should all be spot on.
Keep the ones that get "thin" and use them on leaded houses and for daubbing and scrimming georgian windows. One scrim per this type of house, they start to soak up really well but obviously get damp pretty quick.
Autumn is on ther way and your scrims will get damp quicker and you'll need to change them quicker than in the warmer months so getting them up to scratch now is a factor.
If you keep doing this you will always have a premium batch for detailing that stay drier for longer, a new batch to "break in" by using them on to wipe sills and old ones that get thin after months of use. Keep this process going and before you know it you'll have a range of scrims that suit all jobs....easy peasy.....