MDF is a medium-density Fiberboard that swells with moisture and water. MDF board gets swells after applying the acrylics(water-based) paints, especially thinner formulations used to paint detailed tiny things, creating a fuzzy surface that is very difficult to correct.
You need to seal MDF so that you can paint over it without any difficulty. Here is some guideline about why it is necessary to seal the MDF board.
Seal MDF to prevent Distortion and Swelling
You need to seal the edges and wood surfaces before applying acrylic paint to overcome the frustrating condition. If you are working near water or humidity, you need to apply primer and sealed on all the edges to ensure it does not distort or swell. Even a small amount of water seeping under the MDF board can cause swell if it does not seal properly.
Seal MDF Edges which are Most Absorbent
The edges of MDF boards have the nature to absorb paint heavily. Make ensure to seal the edges properly. To prevent the edges from swelling use an acrylic paint sealer or drywall sealer, or you can use an oil-based primer or glue that works very well to seal the edges. You can use any kind of primer which dry fast in your particular humidity.
So the given points are mentioning about How to seal the MDF edges before painting:-
Sealing MDF using PVA Glue:-
To seal the edges you can spread or spray a layer of thin PVA glue over the edge using a glue spreader, or a pump spray bottle or airbrush, and make sure the glue is as even as possible, without building up any edges. The sealed MDF edge can be sanded when dry, but you should not completely sand it through the glue coating.
Sealing MDF using oil-based filling primers:- oil-based primers are another option you can use for priming MDF edges and sanded down when completely dry.
Sealing MDF using Drywall Compound and Primer:-
Another way of filling the edges consists of rubbing a thin coat of drywall compound into the edges, wiping the drywall compound off as evenly as possible, then sand it thoroughly when dry, before applying a primer.
How to seal MDF Surfaces
1. Seal the MDF using PVA Glue:-
With the use of PVA glue, it becomes easier to seal the MDF surfaces evenly. Most PVA-based carpentry glues require dilution with water to get a thin sealing layer on the MDF. Thin, acid-neutral PVA glues designed for paper and bookbinding work well if applied using a roller, spreader, or spray bottle designed to work thinner glues.
2. Using a paint primer for MDF surfaces:-
Primers used for MDF must be solvent rather than water-based. Stain covering primers can be used for the edges and also for flat or detailed surfaces as the primers have heavier bodies.
3. Using a spray-on Lacquer:-
A colored or clear spar lacquer can be used as a primer on MDF with nice results. Inexpensive craft paints are not that much good to give good results as compared to lacquers that create smooth coatings. Follow the instructions properly while using lacquer when working with them. Always work in a full-ventilated area and try to avoid all heat sources.
4. Using an Enamel Primer:-
When you apply thin coats of enamel primer, it works very well for sealing MDF surfaces and does not build up layers that will drip or run as the enamel dries up. Choose a color of primer which goes well under your eventual topcoat, and always keep the primer layer as thin as possible otherwise thick enamel sprays might cause you to lose crisp detail lines.