When a hollow core door gets damaged, repair (rather than replacement) is possible if you can overcome the two issues that make such a repair difficult. First, these doors are hollow. There is nothing behind the MDF or hardboard surface skin of the door for a patch to adhere to, or to provide support to prevent subsequent cracking of the patch. Second, many hollow core doors have a texture molded into the MDF or hardboard to resemble wood grain. A smooth patch on such a door will stand out simply because the texture does not match.

Damage to these doors can be repaired by the creative use of products that were originally intended for something else. This Instructable describes how to use Insulating Foam Sealant to fill the void behind the damaged area and Silicone Rubber mold material and casting resin to reproduce the wood grain texture over the patch.

The main activities required to repair a hole in a hollow core door are:

Fill the hollow space behind the damaged area with insulating foam sealant to allow filler material to be applied over the hole and to provide strength and support for the completed patch so it does not crack under use
Use epoxy filler material to fill the damaged area and bring it flush with the finished surface
Create a silicone mold of the finished surface texture of the door (in this case, a faux wood grain texture)
Use the silicone texture mold and casting resin to reproduce the texture over the filled and sanded patch
Prime the patches and paint the door

Step-by-step instructions to complete these activities are provided below.

Step 1: Gather the Materials Needed

Insulating foam sealant, such as Great Stuff
Room-Temperature-Vulcanizing (RTV) silicone rubber, such as Aluminite Quick-Set
Epoxy or Polyester/Styrene based putty, such as Bondo All-Purpose Putty
Acrylic or Polyester Resin, such as Bondo Fiberglass Resin
Modeling clay or putty, such as plumber's stainless putty, to prevent leakage when pouring silicone for the texture form
Solvent, such as lacquer thinner or Goof Off to handle tool cleanup and inadvertent drips of epoxy or resin
Masking Tape, such as 3M Blue Painter's Tape
Disposable chip brush and rags
Primer and paint

Step 2: Fill the Void

Use insulating foam sealant to fill the hollow area behind the defect in the door panel. Insert the tube into the damaged area so that the foam sticks to the back of the other side of the door and fills the void completely between the door panels. Inject a liberal amount of foam to be sure the entire area under the damage is filled. This is easier if the door is taken down and laid flat on sawhorses or on the floor.

You may need to dill holes in the door panel to allow foam to be injected if the door is only split or cracked and the nozzle of the insulating foam sealant can't be inserted. Not many of these access holes are needed because the foam expands so much after application.

As the foam expands, it will squeeze out of the holes. This is OK and expected. Do not allow the foam to drip down the surface of the door or you will have a mess to clean up. Use masking tape to protect the door if you have to fill the void with the door in an upright position.

Allow the foam to cure completely (overnight to be safe).

Step 3: Trim the Foam

After the insulating foam sealant has completely cured, it will be quite firm and no longer sticky to the touch. The foam provides enough strength to prevent the damaged door panel from flexing during normal use. If the panel is not supported sufficiently, your patch may eventually crack from the constant flexion and vibration of opening and closing the door.

Trim off all the excess foam that has protruded while expanding and curing (using a knife or saw). Sand the area to be sure it is flat. Indentations are OK at this point, but all high spots must be sanded off.

Step 4: Smooth the Surface

Using an epoxy filler material, apply a layer over the damaged area to fill all low areas and bring the patch level with the surface of the door panel. Follow the instructions on the filler material you are using to mix a batch. Smooth it on using a plastic spatula or similar tool.

It is better to leave the filler a bit high and sand it down to the level of the door panel to avoid having to apply another coat. But if the defect is too deep (more than about 1/4 inch or 6 mm) then multiple layers may be recommended by the filler manufacturer.

Let the filler cure completely (15 to 30 minutes typically) and sand smooth.

Step 5: Prepare the Texture Mold

If your hollow core door is smooth and flat, you are done. Prime and paint the door. But if your door has a wood grain texture, then the smooth, flat patch will still show up badly after painting. So now it is necessary to reproduce the wood grain texture prior to painting.

Find a place on the door that has no damage, where the wood grain pattern is in good shape. The bottom of the door is a good place to consider because it is typically wider than other areas, and you can create a texture mold that is wide enough to cover most damaged areas. Create a frame out of wood and secure it to the door area selected for the texture mold. Clamps or small nails can be used for this.

Use clay or putty to seal the crack between the wood frame and door to prevent silicone from leaking out.

Level the door so that the silicone settles out to an even thickness across the entire frame.

Following the manufacturer's instructions for the RTV Silicone Rubber, mix and pour the silicone into the frame. Vibrate the door for a few minutes to help bubbles escape and to ensure the liquid silicone seeps into all of the wood grain texture lines. This can be done by banging lightly on the underside of the door with a rubber mallet.

Let the silicone cure completely. See the manufacturer's instructions to determine how long to wait. When totally cured, remove the frame and peel up the silicone texture mold. It will not stick to the door. The mold will have a negative relief of the wood grain pattern of the door.

Step 6: Reproduce the Wood Grain Pattern on the Patched Area

Place masking tape around the area where the wood grain pattern will be created. It is best to overlap the existing wood grain pattern a little bit, so your silicone texture form should be a little longer than the patched area. Mix the casting resin according to the manufacturer's directions and apply it in a thin coat over the entire area. This should be a THIN coating...the texture layer is not very thick..

Place the silicone texture form over the resin and press it down firmly. Rolling with a rolling pin (I just used the Great Stuff can) from the center outward will help squeeze out the excess resin. Use a disposable paint brush or a rag to clean up the excess resin that squeezes out onto the masking tape. Once the silicone texture form has been rolled, place a flat board with some weight on it to maintain firm contact with the door.

Let the resin cure completely. Peel back the silicone texture form, and the wood grain pattern will be visible in the cured resin. Any bubbles that get trapped under the silicone texture form will create little holes in the wood grain pattern. I was able to fill those tiny holes with some caulking prior to painting with good results.

Step 7: Prime the Patched Area and Paint the Door

All that is left is to prime the repaired area and paint the door. If you look very closely you can see where the textured patch edges are, but after priming and painting the casual observer will not notice.

Some lessons I learned while using this method to repair 5 hollow core doors:

Make the silicone texture mold large enough. A 10 to 12 inch square silicone texture form will repair just about any door damage you ever run into. The form is reusable, so you only need one to make multiple repairs.
It is possible to create a wood grain texture directly in the epoxy filler putty, but the filler putty is much more viscous than casting resin, so you have to use more force when rolling the silicone texture form into it. I found that for small holes, such as the hole left by a doorstop, epoxy putty is all that is required. For larger areas, such as the fist vs. door damage, the casting resin produced a better repair.
When using the silicone texture form to produce a wood grain finish in either epoxy putty or casting resin, only a THIN LAYER of the material is needed. If the resulting wood grain texture turns out too thick or lumpy, you can always sand it down and try it again. Use the rolling pin technique to squeeze out the excess. Practice does help when using this method.
Goof Off is a great solvent for the insulating foam, the epoxy putty, and the casting resin if you use it before the materials have cured. It is not as harsh as lacquer thinner but still removes the spills from your work (or your fingers).