On Jan 16, 1:28 pm, "SonomaProducts.com" wrote:
If you have seen any of my posts before, you'll know that I usually
think I know it all but apparently I don't and now I have a few
questions. My domain consists of solid wood and the ocassional ply for
all my finished goods type production. I use MDF all the time for
fixtures, etc. but have never incorporated it in a finished piece.

I am building a fireplace surround and mantel for my new home. It
seems best to build some of the parts from MDF. It will be painted.

Q's.

1. Is MDF paintable? It ssems the moisture will screw it up. Is there
some priming method required?


Yes, mdf takes paint very well - at least on what I will call the
"face grain".

On the "edge grain", yes, you should prime it. ordinary joint compound
and a light sanding works well.


2. I seem to recall a variant that has a paper coating. Is that
required?


You're thinking of "MDO". MDO is NOT the same as MDF. MDF is "Medium
Density Fiberboard". MDO is "Medium Density Overlay".

Originally developed for sign painting and concrete formwork
industries, MDO is also used for other applications as a paintable
surface.

It is a plywood product, with a paper face overlaid on the plywood
substrate with an exterior grade (usually) phenolic adhesive. MDO is
designed to have a smooth paint receptive surface. One or both faces
can be faced with paper, and is found in 4' x 8' sheets from 3/8" to
over an inch in thickness. MDO is used for short term signs. MDO is
not recommended for permanent sign faces because it is susceptible to
Warping, Water log, Mold Etc.


3. What about edge prep if you are using the papered version.


MDO can be edge-banded or filled. It is plywood.

4. I will have a scenario where I want to use butt joints. I want to
sort of puttey the end grain (or edge) and fill the gap of the butt
joint and sand it out smooth so it won't show once painted. Could I
just use like sheet rock taping compound or is there something better?


However you join MDF, take care with the fasteners. Use "Spax" screws
or the like. You need a coarse thread with MDF.

Also, be aware that MDF likes to "mushroom", so use of countersinks on
BOTH sides is recommended.