If you are using waterbased paints for the white finish then simply use a water based hardwearing clear varnish over the top. . Ronseal do a huge selection of these in various sheen finishes, satin is usually a popular finish as a gloss finish can be quite reflective. The more coats you put on the harder wearing it will be and it only takes a few hours between coats. I did this on my home office desk and 8 years later it is still hardwearing.