How would you do this?

[Cooler]

I made this table top from two layers of MDF.  I filled the edges with grain filler.  I put a coat of Sealcoat shellac on the machined edges prior to applying the paint.  The edge gold is One Shot gold, a oil based enamel formulated for sign painters.  

The entire top was painted (with a short napped roller) with flat finish latex paint.  I then top-coated it with 4 coats of glossy poly (oil based).  

It is about 18 years old, but lightly used in the guest bedroom.  

The flat finish latex seems to roll out smoother and the shine if from the top coat anyway so I would use a flat finish.  General Finishes "chalk paint" (imitation) brushes out perfectly smooth and I would use that if I were doing this today.

You could just use a bead of paintable caulk for the interior radius.  Wipe it to the radius with your finger.  Once it is painted you will never know if was caulk.  It is the simplest and most fool proof way to do this.  

You are working with MDF so don't let the purists talk you into something more complex.

[cams2705]

I knew someone, maybe many, would question the use of MDF.  I'm leaning towards using it because it finishes better than any other wood type product I can think of.  Maybe it will be a disaster, but I figure it's a half bath that hardly gets used and a good finish should protect it from typical water splatters.  If it fails, I'll use something else.  

John

Have you considered building a form with melamine and pouring a concrete countertop?  You could dye it just about any color and it would be impervious to moisture.

[JGrout]

how about forming it using bondo? 

I think I would be inclined to agree that the cove added to the corner as an applied piece is a good solution, I may however suggest you cut a rabbet in the adjoining pieces to give the cove a bit more support in the corner. Additionally if there is any mismatch either way you would be able to fill or remove material to bring them to a flowing plane.

Joe

edit to add I am fully confident that MDF is appropriate and functional in this application the surfaces will be painted which should be adequate

[shifty]

I would find some MDO instead of MDF. It paints up as good as MDF but will handle the moisture.

[jteneyck]

Thanks.  I actually have some MDO.  The problems with MDO are that it's not dead flat like MDF, you can't sand it much, and the cut edges are harder to deal with.  

John

[Phil Thien]

Are you going to use the moisture resistant fiberboard?

I can tell you, btw, that any seams that aren't mittered will eventually telegraph through your paint. This is a common issue in building speaker enclosures.