I want to paint a portrait with oil on canvas...

There are dirt on the canvas so im thinking of painting black background color to cover up thoae dirt...

Do you think its necessary? Or i should ignore the dirt and start doing my outline first then only the background?

Please help

Oooh its you again! Hi... actually this canvas is those premade oil paintings with numbers and lines for reference...i worked on it a little but decided to change my work due to request..so basically the lines are all over the canvas with a little paint on it..