Flat paints are as porous as a sponge!

Basecoating

Picking the right basecoat is an important step to ensuring the success of your project.  In this case basecoating does not mean priming!  If you are working with bare wood or sheetrock you must prime it first. 

When i use the word "basecoating" i am referring to the base of your faux finish.

This choice can vary depending upon what you plan to do with the wall.  Are you glazing, if so is it a positive or negative?  Are you venetian plastering or do you plan to paint stripes?

Generally, if you paint the wall with an eggshell paint you will have no problems regardless of what finish you are doing.

Why?  

Eggshell paint has a sheen to it as i discussed in this article.  The sheen can be thought of as a protective coating that makes the paint less porous and also washable.  Porousity adversely effects the open time of the glaze so if you are doing a negative finish you never want to use a flat!

The exceptions to the rule: 

I can think of two exceptions that i have not used an eggshell paint as a basecoat. 

1) Venetian plaster - you dont have to use a basecoat for venetian plaster particularly if you are looking for a rough finish.  I like to basecoat in a color matching the plaster color because you use less plaster (and plaster is expensive!)  Also, if you do basecoat you can use a flat or an eggshell.

2) Striping - i once did vertical stripes in a hallway using a flat and an eggshell in the same color.  Beleive it or not it is a dramatic effect!  In this case i basecoated with the flat and then added the eggshell stripes.  The reason for this is becase flat touches up with a brush much better than eggshell.  You inevitabley get some bleeding of the eggshell paint under the tape onto the flat paint.  With a steady hand i can cover up the bleed with the flat and it is unnoticable.