MDF (medium density fiberboard) is used for many building projects, especially those with routed details like dollhouse kits and displays. It is an easy to work material which takes detailing well. Unfortunately it swells with moisture or water, and swells noticeably with the application of acrylic (water based) paints, especially thinner formulations used to paint detailed miniatures, creating a fuzzy surface which is very difficult to correct.
To prevent frustration when finishing dolls houses or other miniature displays or items built from MDF you should seal the edges and the wood surfaces before applying acrylic (water based) paint coatings. If your MDF project will be anywhere near high humidity or any chance of water, it should be primed and sealed on all edges to make sure it does not swell or distort, even a small amount of water seeping under an mdf unit can cause sections to swell noticeably if they have not been sealed.
Ways to Seal MDF Edges - The edges of MDF boards absorb paint heavily. To seal the edges or flat surfaces you can spread or spray a layer of thin PVA glue over the edge with a glue spreader, or a pump spray bottle, making sure the glue layer is as even as possible, with no build up at the edges. The edge can be sanded when dry, but you should not completely sand through the glue coating. Thin layers of oil based filling primers can also be used on the edges, then sanded down when thoroughly dry. Another method of filling the edges involves rubbing a thin coating of drywall compound into the edges, wiping the drywall compound off as evenly as possible, then sanding it when dry, before applying a primer.
Sealing MDF Surfaces
- Seal the MDF with PVA Glue - You can seal MDF surfaces with thin layers of PVA glue. Most PVA based carpentry glues require dilution with water to get a thin sealing layer on your MDF (which can cause the MDF to swell). Thin, acid neutral PVA glues designed for paper and bookbinding work well if applied with a roller, spreader or spray bottle designed to work with thinner glues. Some brands of PVA as a primer may continue to be slightly 'rubbery' under paint coatings and will not withstand wear as well as paint applied over solvent based primers.
- Using a Primer - Primers used for mdf should be solvent rather than water based. (lacquer or enamel, rather than acrylic). Stain covering primers (Zisser makes one in a spray can) have heavier body and can be used for the edges as well as flat and detailed surfaces. For miniatures with fine cut details (1/4 scale siding etc.) these heavy bodied primers may be too thick and may soften detail.
- Using a Spray on Lacquer Clear or colored spray lacquer can be used as a primer over mdf with good results. Use one designed for exterior finishes. Some lacquers create very smooth coatings which inexpensive craft paints will not adhere well to. If you use these liquid acrylics for painting your miniature projects, test them over the lacquer to make sure they adhere well before you use them to topcoat the lacquer primer.
- Using an Enamel Primer - Enamel primers work well for sealing MDF surfaces provided you apply thin coats and do not build up layers which will run or drip as the enamel dries. Choose a color of primer which will work well under your eventual topcoat, and keep the primer layer as thin as possible. Avoid using thick 'one coat' enamel sprays as they may cause you to lose crisp detail lines.