When printing from a 3D printer what’s the best way to paint a finished object
with paint and brush 
Good one @Brett_Beckmann but is there a particular paint that works best, my buddy just got a new 3d printer and when he paints stuff the paints doesn’t come out smooth
Depends on how you want the finished print to look. If you want it perfectly smooth, best to use sandable primer and give it a few coats while sanding it in between. That will get rid of all the layer lines. And then simple spray can for the base coat and what ever colour you want. If you want to use a brush, for the details, you can use any paint type that is compatible with the primer.
Now if you want to paint directly on the print, you need to make sure your paint is not too thin. The little grooves of the layers will wick the paint away and make it very difficult to keep the paint where you want it. Acrylic paint will work, but solvent based paints will work as well. Does not matter much as far as I can tell.
To make it shine you can spray a lacquer over the acrylic paint.
Hope this helps?
Thank you @Lykle_Schepers I will pass it on, it should help