I am making a free standing bathroom cabinet for my niece which she wants painted white. Need recommendations for best filler to use to hide the line where the top and sides met so that when I paint it there is no obvious join line. I am using Baltic birch plywood for the case. Also, I will be spray painting it with a standard latex semi gloss.
Not used to painting things I make.
Miter the top to the sides, or put the top panel between the sides to eliminate the line. Most filled cracks return eventually, especially in a humid environment.
You could also make the top with an overhang so it becomes a detail instead of a line.
JB weld kwik wood, an epoxy paste will bond better and resist better than the single component air dry fillers.
I agree rethink your design to eliminate such a unnecessary seam, even if it's as simple as having the sides go full height and the top fit in between, you don't mention how tall the cabinet is. No filler won't eventually fail.
Also I recommend you rethink your choice of paint to something more suited to cabinets and furniture rather than ordinary latex house paint.
Agree with Esch on both points.
I like Ben Moore Advance for painting cabinets and bookshelves. It's much harder and more durable than regular latex paint. It also gives a better finish, akin to spraying, with just a brush. And it's also water based and water cleanup.