First I tried to refine PLA printed parts:

• printed on slower speed
• sanded
• painted with XTC-3D coating.

And the same keycap using ABS and acetone bath

I tried many techniques, it turned out that the best one is the simplest one:

• Take a food box.
• Put the paper towel slightly wetened with acetone to the bottom of the box.
• Put a piece of aluminium foil atop of towel in its center.
• Put ABS parts on the foil.
• Close the box with hermetically fit cover.
• Wait 15 minutes.
• Profit.

Despite some curling, I chose ABS parts for their slick ivory look and feel:

So now my keyboard looks like this: