How do you remove mildew on the inside of a Sunbrella cover? Is there some way to lay it out in the yard and scrub it? With what?

Sounds drastic.....but Bleach and water do a great job, I use nothing else on mine

Try a solution of 60% water and 40% bleach to start. Let the solution soak in for 5-10 minutes and then scrub with a deck brush. First test in an inconspicious place. You can also purchase Mildew Stain Remover, but it is expensive, Approx. $12.00 at West Marine or Boaters World.

Make sure you re-treat it with 303 waterproofer when you're done. Recommended by Sunbrella.

Be sure to wash in hot water before treatment or it won't work. I washed mine in the washer with detergent and it took almost all of it out but now have to rinse and treat .

Red

I haven't found a washing machine big enough to wash/rinse a 40 foot boat cover yet I use Woolite with a brush ,its mild and want leave a lot of soap residue

I spread mine out on the driveway, used laundry detergent, a deck brush, and sprayed it off with a hose. Smelled good too.