Hi

How come when you paint MDF with emulsion, one coat covers it perfectly, but
if you try gloss paint without a primer it just soaks into the board?

Doesn't make sense to me, it should be the other way around.

Dave

david lang wrote:

How come when you paint MDF with emulsion, one coat covers it perfectly, but
if you try gloss paint without a primer it just soaks into the board?

Emulsion?

Doesn't make sense to me, it should be the other way around.

Why?

Weatherlawyer wrote:
david lang wrote:

How come when you paint MDF with emulsion, one coat covers it
perfectly, but if you try gloss paint without a primer it just soaks
into the board?

Emulsion?


Yeah! you can use emulsion as a sealer and primer on MDF.

Doesn't make sense to me, it should be the other way around.

Why?

Weatherlawyer wrote:


Emulsion?

Emulsion paint, like wot you put on walls.

Doesn't make sense to me, it should be the other way around.

Why?

Because MDF is highly porous and you would think it would soak up water
based paint. Gloss paint being solvent based (mainly) should sit on top &
cure into a film. But it don't!

Just wondered why?

Dave

Because MDF is highly porous and you would think it would soak up water
based paint. Gloss paint being solvent based (mainly) should sit on top &
cure into a film. But it don't!

At a guess (but based on some science) MDF is bonded with organic resins,
which are likely to be hydrophobic (i.e. water-repellant). Hydrocarbons like
the white spirit in "real" gloss paint will wet it and be drawn in by
surface tension, while water based paint doesn't? Absorbtion into a porous
medium depends on a balance between pore size, contact angle and surface
tension. This is how Gore-Tex and similar breathable fabrics work, also why
most insects don't drown easily in water spray.

In article ,
says...
snip
Because MDF is highly porous and you would think it would soak up water
based paint. Gloss paint being solvent based (mainly) should sit on top &
cure into a film. But it don't!

Just wondered why?

Apart from the solvent[1], emulsion is mostly pigment and filler with a
bit of binding agent, gloss is mostly binding agent with a bit of
pigment and filler. So when the emulsion soaks in it leaves more solids
behind.

[1] Strictly speaking emulsion doesn't have a solvent because it's not
a solution ...

Rob Morley wrote:

[1] Strictly speaking emulsion doesn't have a solvent because it's
not a solution ...

I know what you mean re emulsion, but isn't water the 'universal solvent'?

Dave

"david lang" wrote in message


Rob Morley wrote:

[1] Strictly speaking emulsion doesn't have a solvent because it's
not a solution ...

I know what you mean re emulsion, but isn't water the 'universal solvent'?

Oil in oil based paints are not solvents either. Striclty speaking they
are vehicles. Emulsifiers. Allowing you to work with the rest of the
ingredients.

The reason that oil based paints don't cover MDF so well is that the
bottom colour -brown, is grinning through. If you gave the board the
same thickness of material in the vehicle as you did with the acrylic,
you would be putting on a very thick paint or many more coats.

Oil based paint is used for fine, cabinet-quality finishing. Acrylic
isn't. In fact the thickness of acrylic paints allows for the amount of
moisture that the surfaces it is used on is going to absorb.

And that is why it is used as the primer on porous surfaces.



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david lang wrote:
Rob Morley wrote:


[1] Strictly speaking emulsion doesn't have a solvent because it's
not a solution ...


I know what you mean re emulsion, but isn't water the 'universal solvent'?

Dave



The solids in an emulsion are water "borne" rather than water soluble. A
bit of a fine line anyway when you think of milk being an emulsion.
Anyway, IME mdf isn't very porous on the faces.