We always use rolled up towels snug against the bottom of the frames, it also protects against chemical splash which might damage some kick plates on the bottom. There are other things you can use but long beach towels seem to work nicely.
Paint Shields, Have customer towel behind door, duct tape but pre tape with painter tape and the duct tape on top of the painters tape. Also angle your nozzle away from door.
I think I'll try the towels or paint shield. Or both. It's difficult when it's super dirty right up to the door or if there's a piece of gum right there. It's almost impossible to not get water under the door!