Painting MDF problem?

Question

I'm painting some MDF boards to make a desk. Since the surfaces are smooth I thought the paint would be smooth when it went on. (i applied with a 3/8 roller). Should I use a thinner nap , apply with a brush , does the paint matter (right now its just primer)?

Answer 1

for something that smooth use the shortest nap roller they have and get a lint free one

Answer 2

MDF, is merely compressed paper pulp.   It is usually used as a core material for Formica, or some other laminated plastic covering.   It is a very absorbant material.   Painting, requires a seal coat, made to work with MDF.   Regular paint, will be absorbed into the material, and will usually absorb differently in different areas, making a rather blotchy surface.    When painted, the "grain" of the material, such as it is, "fuzzes" up and sanding is difficult.   Applying paint with a roller, will only serve to "creat texture" to the surface.   The paint usually will not "flow out" and dry to a smooth surface.  Using a "thinner nap (roller)" will not provide you any smoother surface.  brushing will give you a surface where the brush marks will be seen.   You indicated, ..."right now its just primer", well, you will most likely have to apply several coats of finish to the material, to ensure a good seal, especially on the edges, before your final coat will look right.  One way to help ensure the material is sealed, prior to any paint work, is to mix a "Sizing Solution" consistiing of a very thinned "watered down glue" mixture.   Paint the surface with the Sizing, be sure the edges are coated, and allow to dry.   After drying, lightly sand the surfaces, and tack rag the dust.   The "sizing solution" will assure a better seal.   You may have to "size" the edges several times to ensure a good seal.   Allow the second coat to dry, re-sand the surfaces, and after tack ragging the dust, now you could try to paint.   For large surfaces like the desk top, you can use a fine "short nap" roller, but be prepaired, it will never be "smooth" and again, brushing will still show marks.   If you want to have the best finish possible, try and get access to a sprayer.    You'll get a better finish using the sprayer.