The niche will be used as a shelf, which material?

[andreascarlsson]

I am building a new laundry room in the basement and have built new returns for the basement window. The walls and ceiling of the return are made of plaster, but due to lack of space, I chose only 10 mm OSB as the "base plate" (the idea was that OSB holds more than plaster since the return should be able to be used as a resting area (aka "shelf")).

Then I applied a layer of filler, but at the same moment I did this, I realized that the OSB will probably absorb the moisture in the filler, and it simply will never become really good. The photo shows how it looks when the filler has dried and is now ready for the next layer. It looks okay and can probably withstand a few more layers of filler, but then I thought there are many smart people on Byggahus who might know a better way!?

Maybe a thin (at most 3 mm) MDF or laminate board, or something else, might be a better solution than filler?

But I also want to reinforce the edge out towards the room, so maybe it's just as well to fill the whole surface?

Option 1: Fill until it becomes good.
Option 2: Lay a laminate on top, ignore the edge.
Option 3: Lay a board on + reinforce the edge.
Option 4: Something else...

Maybe I should have laid a 10 mm pre-lacquered MDF from the start, but that's not how it turned out. 😝