The brushes that used to work in a small sized painting just will not work the same in a larger painting. It is simply because the scale is so much larger! That is why if you have been used to brush sizes 6 or 8 it is time you started using sizes 16 or 18. This is especially true if you are trying to create a portrait from photo. In such cases delicate elements like face, etc. will need detailing and fine finish.
Increasing the paint amounts
Most artists seem to forget that the amount of paint that would normally work on a small painting will cease to provide effect on a larger canvas. Therefore if you have been using a small bottle of acrylic paint on a 8x10 inch canvas then you will need 5 times more paint for a canvas which is 5 times larger. Most seasoned artists who create portraits from photo know that there is a much larger effort and quantity of paint needed in large sized canvases.
Create the swatch first
It is always a good idea to start small and work bigger. That means it is better to create a scaled down version of the painting first before proceeding to the larger sized one. If you are creating a portrait from photos then you need to probably create a small 8x10 inch piece before moving on to a 24x30 inch canvas. You need to paint even the small portrait from photo outdoors as it will provide you an idea of how sunlight will affect the outcome of the painting.
Filtering the sun
The sunlight will have a natural tendency to filter through the cloth. This can affect the way you are able to see your painting. To prevent this from happening just try putting a lawn bag made of plastic behind the cloth. This makes for an opaque covering. This way you will get a realistic view of the paints on the portrait from photo.
