If you do acrylic on canvas or large paper, then you may also need a 10-12.
Better grade synthetics are made by Lowe-Cornell or American Painter, but you
can find good deals with other brands.
To recover a ruined acrylic brush, I try soaking the bristles in alcohol and
begin to work the paint off with my nail. Then rinse with soapy water. Be
careful that the brush ferrule does not sit in the alcohol. It may loosen the
glue holding the bristles.
I keep my brushes separated into ones appropriate for certain media. Keep your
ruined ones for work tasks like gluing,etc. Watercolors can do well with a
school grade squirrel brush. I have 2 sizes for Elem. One about size #6 and a
2. Chubbies are for k-2 and I use them mostly with Tempera. I find I can do
most with these if we take care of them. Good Luck!