What are the most opaque acrylics suitable for painting a design on gloss emulsion paint on an old milk churn
Some pigments are naturally more opaque than others and Artist quality acrylics will offer you the better Pigment to binder ration and covering power than a student quality. Choose your colours carefully using only naturally opaque colours.
You can also increase the opacity of acrylic by adding a body which means that you can add an opaque white to your colour and this will increate the opaqueness of the paint. You do have to be aware that too much white can alter the colour by making it lighter.
There are mediums you can add to acrylic to increase the Opacity of the colour:
Liquitex makes a Matte Opaque Gel Extender Medium which increases the Opacity of Acrylic paints and will keep the paint wetter for longer.
Alternatively, there are wonderful Opaque paint pens like the Posca markers which will offer good opaque qualities in a pen form.