Medium Density Fiberboard (MDF) is a synthetic building product, mostly used in cabinetry. It is made by mixing glue or resin and wood pulp under intense pressure. MDF is preferred to plywood because of its high resistance to warping. It is also versatile and cheaper than wood.
The downside of MDF is its poor resistance to moisture. MDF cabinets warp when exposed to moisture or high humidity. Hence, MDF must be stored in a dry place before use and sealed after construction.
This guide recommends the best products to seal MDF to protect it from moisture and other contaminants.
The Best Products to Seal MDF
You need to seal MDF edges completely to prevent warping and swelling. You may use any of the following products, but ensure that you follow the right procedure. A tiny drop of water can mess up the whole MDF structure.
1) Paint Primers
Paint and primer manufacturers have special primers for wooden walls and wood-based structures. For instance, Zinsser has a special primer for MDF structures. The point to remember when considering primers for sealing MDF edges is to avoid water-based primers.
Oil-based and enamel primers are perfect sealers. You will need several coats of primer to seal MDF edges well. Another important consideration is that heavy primers may interfere with fine-cut details on structures.
You can use any of the alternatives in this guide to preserve fine details on the edges. Otherwise, paint primers are easy to apply on flat surfaces and edges. Allow each coat of primer to dry before spraying or painting a new coat for the best finish.
2) PVA Glue
PVA carpentry glue seals MDF surfaces and edges perfectly. You can use a spray bottle, brush, or glue spreader to apply the glue on the surfaces. Apply in even layers and smoothen out any lumps, especially on the edges.
Allow the glue to dry up, and gently sand the glue coat. PVA glue comes with instructions for use. For instance, some brands require you to dilute the glue with water to achieve thin coats. The diluted glue is easy to spread, but it may cause the MDF boards to swell.
Choose brands that do not require dilution with water. In addition, the glue should withstand wear after painting. Some of the trusted brands for PVA adhesives include Lineco, Gorilla Glue, Elmer’s, Krazy Glue, and Titebond.
3) Drywall Compound and Primer
The method is simple. Apply drywall compound on the MDF edges in thin layers. Use a clean, dry cloth to wipe off excess drywall compound for a smooth finish. Leave the drywall compound to dry and sand. Apply an MDF primer of your choice.
Applying drywall compound on the edges before the primer gives longer-lasting results than a plain coat of primer. Choose an oil-based or enamel primer to coat the boards to prevent distortion.
4) Lacquer
Lacquer as an MDF sealer is suited for outdoors. You can opt for colored or clear spray-on lacquer. The product requires extra caution because it is highly flammable. Avoid spraying lacquer neat heat sources.
An additional tip when choosing spray-on lacquer is to avoid brands that create overly smooth layers. You need the paint to hold after sealing the MDF boards. Test the lacquer primer and paint on a small section before working on all surfaces.
5) Enamel Primers
Enamel primers work well when applied in thin layers. You need to test the primer on a small section of one board because enamel primers build up easily. The primer may form unwanted lumps or drip as it dries.
Thick coats do not only run over the edges but also hide important details on the structure. Spray the primer thinly as you wipe off any runs before it dries. Give it enough to dry before painting.
Always consider the topcoat paint’s color when choosing an MDF undercoat or primer. The colors should blend perfectly. In addition, the paint should adhere strongly to the primer or any other sealer you choose. Test the sealer and paint first to be sure of the results.
6) Varathane
Varathane is a tougher MDF sealer than other sealers. It is perfect for exterior MDF structures that are constantly exposed to rain and sunshine. If your structure is sitting under direct sunlight, order varathane with UV protection.
Some homeowners use varathane when other sealers are ineffective. However, you can use it as a first choice to coat exposed MDF surfaces.
Varathane is effective and long-lasting. However, it can take a long time to dry and seal the surfaces completely. Each coat of varathane takes at least 72 hours to cure and dry completely.
Keep the drying times in mind when choosing this sealer and allocating time to your project. You may not experience the inconvenience if the project is outdoors, but it will take you several days to complete the project.
7) Paint
Paint is enough protection for small MDF surfaces, especially for indoor structures. MDF creates flat and slick surfaces that are easy to paint. However, you need to sand the edges for the paint to hold.
Choose enamel or oil-based wood paint and apply at least two coats. You can sand in between the coats, but ensure that the first coat is dry before sanding or applying the next layer. Apply a third coat if the paint seal is still too thin.
It is important to specify the type of paint you need for your project when ordering paint. Some paints work well on traditional wood but fail to hold or withstand moisture on MDF surfaces.
8) Sanding Sealer
We have one more option for you to consider before the final decision. Sanding sealer is ideal for all MDF surfaces. It is an extra thick lacquer with dense particles. The thick particles easily build up on the surface and edges.
Builders sometimes spray sanding sealer onto cabinet boards and doors before installation. You can apply it on installed cabinets and other MDF boards before painting them. Ensure that you cover the edges properly and allow enough dry time, given the sealers’ density.
Important Tips to Remember When Sealing MDF
MDF sealers preserve its strength and prolong its service life. However, moisture can penetrate even the best sealer if you do not apply it properly. You need the following tips in addition to choosing a quality sealer.
1. Elevate structures in rooms prone to flooding
Coating MDF surfaces with the best sealer does not eliminate the need to control exposure to water. You cannot seal the cabinets and forget about them. If you are adding new MDF cabinets in your garage or underground storage unit, consider elevating them.
Do not wait for a flood to restructure. Remodel the rooms prone to flooding and then seal the MDF and other wooden structures.
2. Proper storage
Long-lasting MDF structures start with strong boards. Sealing MDF surfaces that have been exposed to high humidity or water does not help much. Store the boards in a dry place during the construction process.
If you have limited storage space, order the boards when you need them for construction.
3. Fix flaws first
Flaws on MDF boards, such as dents, rough joints, and screw or nail holes, do not disappear with sealing. The flaws will be visible even with several coats of sealer, primer, paint, and top coats.
Fix the flaws with a wood filler first. Sand the surface when the filler dries and start the finishing process. Sometimes, the only way to avoid flaws is to replace an MDF board with a new one before finishing.
4. Don’t rush the process
We cannot talk about this part enough. Sealers take time to dry. Even normal wood paint or primer requires time to hold on MDF surfaces and dry. The slick surface may cause you to assume that finishing MDF structures is fast and easy.
Follow the manufacturer’s recommendations on the dry times. It helps to wait a few more hours to be sure that the sealer is dry before painting. In addition, spraying or applying the sealer in a hurry will cause it to run over the edges and leave ugly bumps.
5. Pay special attention to edges
The real test of your sealing skills is on the edges, especially those with fine-cut details. Achieving an even finish on a flat surface is easy. However, edges require special attention to avoid runoffs and lumps when working with heavy paints or sealers.
Sanding helps you to keep the edges sharp and smooth. Another tip is to seal in thin coats and to avoid over-sanding in between the layers.
Conclusion
You can seal MDF structures before painting with paint, primer, PVA glue, a sanding sealer, varathane, or lacquer. The right sealer to use depends on the type and location of the structure. Thicker sealers with UV protection are recommended for exterior surfaces.
The best approach is to apply sealer in thin layers and sand in between layers for a smooth finish. Thick layers hide the fine details on the edges. In addition, the structure should be protected from flooding and constant exposure to rain or running water.
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