The phrase "mainly work on" is correct and usable in written English.
It is often used to indicate that one's primary focus of effort is directed toward a particular goal or activity. For example, "I will be mainly working on expanding my professional network this week."
I mainly work on the internet now, on the Comment desk, with a rather more diverse bunch of folk.
I mainly work on the business-and-industry side of things, but all of us get involved in everything.
Existing Web session tracking techniques mainly work on huge web server log files, which limit their dynamicity and capability.
This paper is mainly work on a new design of float and the analytical analysis of its performance.
The content related to the development of commune and brigade industries in the "May 7th Instruction" states, "Peasants of communes should mainly work on agriculture (including forestry, animal husbandry, side-line production, and fishery) but should also learn some knowledge about military, politics, and culture.
Technically speaking, each picture starts with a pencil drawing and I mainly work on heavyweight cold press paper so it soaks up the paint really well.
Although he couldn't divulge the details about possible partner breweries yet, Stuart said the group is "mainly working on paperwork" and hopes to launch in more than one place in the country.
About 3,000 United Nations staff members are in Myanmar, mainly working on alleviating poverty.
He was mainly working on short stories then; gamely sending them out and absorbing the rejections.
Ms. Koenig spent the 1980s and most of the 1990s at the Met, mainly working on the directing staff.
He served in the Office of Strategic Services and the State Department, mainly working on the reconstruction of German democracy, and taught at West German universities.
