MDF filler

spose

hi, I made some shelves recently for a kids room. All mdf, double 18mm. It’s been up a couple of months and just noticed that the filler over all the side screws seems to have expanded. I used the green Ronseal 2 part wood filler and have used on other projects and never seen this. Any thoughts for why this time and so consistent for every side?

dathi

mdf was probably damp when you made the shelves and has dried and shrank while the two part filler hasnt

spose

I was thinking that too since it had been in the shed for a bit first but there’s no other signs of shrinkage on the paint or joints or anywhere