How can I seal subfloor gaps before adding a substrate? [closed]

Question

I installed a new subfloor sheathing in my second floor half bath. The edges are between 1/4 and 1/2 inch from the wall. In a few spots, the wall edges are not totally smooth due to poor removal of previous rotted subfloor and uneven blocking (jagged edge that was not cut or sanded smooth). Basically there are potentially access points for liquid materials to enter the void (don't know the technical term here, aka "the subfloor abyss")

1. What can I do to seal off this gap so I can pour some self leveling compound?

   1. Mildew resistant caulking?
   2. Wood filler?
   3. Mesh tape + some kind of compound?

2. Backerboard or DITRA for a floor tile substrate?

Answer 1

If the 1/4" to 1/2" gaps will be covered with baseboard and shoe molding later, then strips of 3"-5" wide plywood, wrapped/covered in a plastic painter's drop cloth, wedged into the gap will create a fairly tight seal until the leveler dries.

Shims between the strips of plywood and the wall (with the wider part of the shim downward; narrow end up, so the top edge of the plywood can lean against the wall...if it wants) will help ensure the plywood is snug against the subfloor.

Even if a little the plastic somehow gets stuck within the dry leveler (which happens), a cutting blade can trim it.