How to Fix a Hole in a Hollow Core Door

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Introduction: How to Fix a Hole in a Hollow Core Door

About: Builder / Clinician / Analyst / Writer now trying to become a maker.

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:

  1. 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
  2. Use epoxy filler material to fill the damaged area and bring it flush with the finished surface
  3. Create a silicone mold of the finished surface texture of the door (in this case, a faux wood grain texture)
  4. Use the silicone texture mold and casting resin to reproduce the texture over the filled and sanded patch
  5. 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).
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    1 Questions

    Great instructable, thanks! I plan to try this to fix the jole inthe non skid area of my boat deck.

    But to get it to look so good, you must have matched up the wood grain lines in the mold with those in the places where it joined the undamaged area when you tranferred it?

    How did you do that??

    Actually the lines do not match up exactly. I chose a place on the bottom of the door where the texture looked very close to what I was repairing, but positioning the texture form where each tiny wood grain mark lines up is impossible. As I mentioned, if you look closely you can see where the texture patch was added, but after painting the patch blends in well. People walk right past it without noticing it. I paid careful attention to the places where the texture overlaps, scraping any high parts and rough edges to blend in. My pocket knife used like a cabinet scraper was perfect for this little bit of cleanup.

    19 Comments

    Isn't all this more expensive than a new door? Great Stuff is at least $7 then the Bondo is at least $10. A textured door is about $30.

    2 replies

    The only place I can find a door similar to mine for that kind of price is the local Habitat for Humanity Re-Store which sells them second-hand, already mortised and drilled for some other jamb. Remember also that I had 5 door surfaces to repair, and there is enough product in the Great Stuff and Bondo cans for probably 20+ door surface repairs. In addition, these are great products to have around for other projects, so the cost is amortized across all projects you use them for. Hanging a new door correctly in an existing jamb is not an easy DIY task. So the total cost of the new door has to include the skilled labor time (yours or someone else's) that it takes to hang it.

    Where? I'm actually looking for a new door, but prices are nowhere near $30 from what I've found at Lowe's and Home Depot.

    Bondo All-Purpose Putty is absolutely NOT EPOXY. It's polyester resin autobody filler - it gives off lots of fumes, uses a nasty catalyst to cure (MEK). Bondo Fiberglass Resin is NOT casting resin! Casting resin is Acrylic Resin with a low viscosity for pouring. The Bondo stuff - no need for trade names here, it's autobody filler, has fiberglass strands in it and is really meant to not flow. So both of those materials are types of autobody filler. Personally would not use a fiberglass resin to finish.

    1 reply

    As you and zap88 have pointed out, Bondo All-Purpose Putty is a polyester/styrene-based product, not actually epoxy. I have changed the description in the instructable. But the Bondo All-Purpose Putty that I used is not the Bondo glass-reinforced filler that you refer to. According to Bondo's marketing literature (available: https://multimedia.3m.com/mws/media/1283791O/bondo-product-selector-guide.pdf ) it is a "Unique thicker formula designed for wall, furniture and other home repairs". The uses they list include "Drywall Repair, Concrete Cracks, Metal Gutters, Doors, Siding, Brick & Stone Repair, Furniture Repair". The odor of this putty was quite mild, although I did the repairs out in my garage with the doors open. I think Bondo intends this product to be a competitor to fast-hardening drywall compound and floor fix products. It would certainly work well for that. In this instance, including the Trade and Product name for the material I used is important in the instructable because there are similarly named products that would not be as suitable, as you point out.

    The Bondo Fiberglass Resin was a little bit thicker than acrylic casting resins that I have used before, but I was working with the door flat on sawhorses, so a thinner resin would also be easily controllable for this purpose. I have never tried casting Fiberglass Resin material in a deep mold, so in that case an acrylic casting resin might be a better choice, I don't know. But either one could work for this thin layer to copy the wood grain texture.

    This was helpful to learn. Thanks.

    How would fix a hole in a stained and sealed hollow core door?

    1 reply

    This method works for paint grade doors because the resin texture needs to be painted to match the rest of the door. A hole in a hollow core door that has a wood veneer that has been stained and finished would be much harder to repair satisfactorily. I would either replace the entire door skin and refinish it or, more likely, buy a new door and stain and finish to match. I know of no tricks to make that easier.

    Very clear and well-written article.

    I especially liked the wording of the description of materials, for example, "Epoxy Putty, such as Bondo All-Purpose Putty." This gives a good example of what might be used, while suggesting that other materials might be substituted depending on what one is familiar with or is stocked by their favorite dealer.

    One minor correction: Bondo is a good family of products, but it is not "epoxy". It uses a polyester chemistry.

    1 reply

    Thanks, zap88. I did not know Bondo All-Purpose Putty was not epoxy. It does make sense because it was nice to sand down when I had to. (Yes, my first try was too lumpy and I sanded it off for a do-over). Epoxy might be so hard that sanding would not be so easy. At any rate, the Bondo was perfect for this purpose.

    I went to the HD website and priced the materials at $73 before tax. Then I priced a new door slab: Masonite 30 in. x 80 in. Textured 6-Panel Hollow Core Primed Composite Interior Door Slab Model# 16474 $31.98. (The smooth door was about four bucks less). *

    None the less, your craftsmanship is to be envied. One note about the foam filler/sealant, however. I would think the Window and Door variety the first choice for such an application as the Standard stuff has been known to move window and door jabs as it expands (making it difficult (impossible?) to open the window or close the door.) It also does well in moist environments (adhesion-wise) I've been told - so, spritzing a little water into the cavity to be filled should make for an even firmer fix.

    * replacing the door would require matching and mortising hinge locations.

    1 reply

    That is a very good point charlessenf-gm. If only one door in my house was damaged, I may have decided to buy a new one and re-hang it (I have the tools and experience to hang a door, but not everyone can do that). But I had 5 damaged doors and some Bondo and Great Stuff left over from other projects. I was not sure this would work so well when I started, but was pleased with the results. I was not aware that standard Great Stuff could expand so much that it can warp window and door frames. I think it worked fine here because there is so much room for expansion in the hollow door and out the damaged hole. The door skins did not bulge out.

    Great instructable I wish I had know this a few years ago.

    Great instructable.

    Great Stuff (foam in a can) is really great stuff!

    Man, you are a genius.

    Yes, well done on the mold!

    Brilliant Instructable. Especially liked the mold of the pattern. You have no idea how I've tried to match the grain by hand. Oh, I've done it, but not as excellently as you did.

    IF I CAN DO IT.pngHIGH 10.jpg

    Great tutorial. I wish that I had seen this because I renovated my house earlier this year.