Poster colors are water-soluble and can be reactivated. They are also relatively opaque, compared to other watercolours. But even though this type of paint might seem suitable for glazing and layering, you have to be careful when doing so, since the new colour can easily mix with the underlying one.
Like gouache and watercolours, working with poster colours on rougher types of canvas (yours is "medium grain") will cause the grain to stand out as the pigment particles will condense in the deeper parts of the texture.
I don't think there is a way to prevent this, but you can dab or smudge the surface of larger areas slightly before the paint has completely dried and settled, especially if and when you're blocking in your composition.
The trick is to work quickly and use little water.
As a side note: depending on the manufacturer (and, if one looks at translations, on country/culture) poster colours may or may not be synonymous to gouache. They are very similar, in any case, and you can use that term to find more and specific information online, if you want to.