How to Waterproof MDF  For Bathroom and Outdoor Use

Medium-density fiberboard, or MDF, is versatile and inexpensive, which makes it ideal for shelving, cabinetry, furniture, and molding.

But as it's neither water-resistant nor waterproof, the board can still warp and swell if exposed to humidity and water.

You should, therefore, never use MDF outdoors or in an area where it would get wet or be constantly exposed to dampness.

That said, if you need to "waterproof" MDF so that it can withstand moisture better in high-humidity areas like bathrooms, there are two approaches you could take - Make the piece water-resistant yourself or use Moisture Resistant (MR) MDF.

1- Making MDF Board Waterproof for Outdoor Usage

MDF boards can be waterproofed for outdoor usage by adding a wood sealant, varnish, stain, or moisture-resistant paint that will hold up well against moisture and humidity.

It's best not to use waterborne polyurethane as it might not adhere properly to the MDF surface.  Start with a latex primer instead, or one explicitly designed for MDF.

Use a 3/8-inch paint roller for the primer and a brush for any areas difficult to reach.  Let the primer dry completely before proceeding (this should take about six hours).

The second step is to apply the sealant or water-resistant paint with a 3/8-inch roller. Use a brush to get into the corners and edges. The layer should dry within four hours.

The third and final step is your last layer of either paint or sealant; give it at least three days to ensure MDF won't absorb any moisture.

2- Purchase Moisture Resistant (MR) MDF for Bathroom and Kitchen

The most convenient option is to buy MR MDF, which operates well in interior spaces where moisture might be an issue.

In contrast to regular MDF, moisture-resistant MDF is still easy to sand and mill, takes well to most adhesives, veneers, and paints, but is made with a water-repellent resin.

It’s great for kitchens, bathrooms, boilers, and laundry rooms-places where humidity gathers. Although it costs more than standard MDF upfront, the long-term investment in its moisture-resistant feature pays off.