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
Why?
Yeah! you can use emulsion as a sealer and primer on MDF.
Emulsion paint, like wot you put on walls.
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
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.
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 ...
I know what you mean re emulsion, but isn't water the 'universal solvent'?
Dave
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.
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.
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