Mosaic Mailbox Jill Gatwood

Mosaics on Steel Mailbox Using Silicone Adhesive

Artist Jill Gatwood has emailed me her procedure for using GE Silicone II to mount mosaic tile to steel mailboxes, and it is outlined below.

Jill’s instructions have convinced me that there are enough mosaic applications for silicone adhesive that we should sell it. Note that we still recommend thinset mortar or Weldbond for mosaics on architectural surfaces such as backsplashes, but for projects such as mosaic  mailboxes or glass-on-glass mosaics, silicone adhesive is preferred.

Jill’s steel mailbox instructions are fairly complete and include recommendations for purchasing the right type of mailbox for the project and modifying it as needed.

Steel Mailbox Instructions

1. Selecting The Mailbox

Jill says that a steel mailbox from the big box home improvement stores or a hardware store will work but you have to check it and make sure the metal is strong and doesn’t flex. (In practical terms, this means you should buy the mid-grade or premium model and not the one made for the bottom of the market.)

Jill recommends getting one that has ribs to strengthen the frame if you need an XL size mailbox.

Mailbox Door Reinforcement

Mailbox Door reinforced by a steel plate held inside the door by bolts inserted from the outside.

2. Modify And Sand The Mailbox

Reinforce Door

Jill reinforces the door with a metal plate because otherwise the door is most likely to flex and pop off tiles.

Remove Flag

Remove the flag, which hides the mosaic design. The tile would also prevent the flag from turning up or make it difficult.

Sand Select Areas

The mailbox should be sanded all over, except for the bottom rim and any other part that will be exposed and not tiled.

  • Leave the bottom inch of the mailbox un-tiled. This area is a bit weaker and tiles can be lost during mounting.
  • Leave an un-tiled rim where the door opens and closes.

As always, we recommend using pieces for the indestructible belts made for belt sanders instead of quickly disposable sand paper. (The price is about the same, but one lasts forever compared to the other.)

Silver Sharpie Draw Mosaic Pattern

A Silver Sharpie is used to draw the mosaic pattern directly on the bare steel.

3. Draw The Pattern

To draw your design on the box, Jill recommends using a silver Sharpie pen or one of those red crayon pencils that peel down with a string to expose the tip. A silver Sharpie pen is used instead of a regular black Sharpie because it shows up better on the bare steel.

She recommends that you personalize your mailbox with your address number or name so that it is less likely to be stolen.

Mailbox Mosaic Adhesives

Mailbox Mosaic Adhesives

3. Select An Adhesive

Avoid using a silicone caulk chosen at random as they might vary in terms of fillers and cracking over time or hardness or some other property. GE Silicone II seems to have the most fans online for mosaic work. Jill often uses it for her mailboxes, and we have used it in-house for glass-on-glass mosaics.

Jill says that E6000 can be used, but we avoid it due to fumes and toxicity and stick to silicone.

Mosaic Mailbox Rear Panel

Mosaic Mailbox Rear Panel

4. Tile The Mailbox

Tile the mailbox one side at a time and rotate it as needed to turn each side up for tiling. Silicone hardens faster than white PVAs such as Weldbond but take care not to turn the mailbox before the silicone hardens and risk shifting the tiles on the side resting on your work surface.

Painters Tape Grouting

Painters Tape is used to cover un-tiled areas before grouting.

5. Grout The Mailbox

Let glue cure for a day or two and then grout with sanded grout. Usually a dark grout looks best. Jill uses painter tape to cover the parts that won’t be grouted (otherwise you have to clean grout out of the holes and the door mechanism).

Let the grout dry for a day or two and then seal the whole mailbox with 511 Impregnator Grout Sealer. This is a wipe-on/wipe-off product that can be purchased at Home Depot.

Note that sometimes these water-based grout sealers will have negative reviews online. I speculate that most of these negative reviews are from people who used material that froze during shipment. That  is why we don’t see the sealer in our online mosaic store and allow people to purchase it locally.

I think that the remaining negative reviews are from people who didn’t buff off the excess sealer.

6. Mount the Mailbox

Post: If you don’t already have a strong mailbox post, use a 4” x 4” wooden post or a hollow metal post 3 ½” square. Sink post into the ground with concrete.

Mailbox platform: I recommend using a Universal Mounting Bracket available at Home Depot/ Lowe’s/most home improvement stores). It includes instructions and screws, etc.

A second option is to secure a 6″ x 18 1/2″ block of wood to the mailbox post. Attach the mailbox to the wood base with 1 5/8″ coarse-thread drywall screws through the side holes of the mailbox.

7. Maintenance

Because mailboxes are exposed to the elements and get a lot of wear, inspect the condition every few months, especially the door. Because the door gets a lot of use, you may need to occasionally patch the grout or even replace a tile or two there. Re-seal with grout sealer every six months or so for maximum life.

When To Use Silicone Adhesive

For years I have recommended that people NOT use silicone caulks as an adhesive for mounting mosaic tile, and I did so for several reasons.

The main reason I discouraged silicone was that people were using it for architectural surfaces such as bathroom backsplashes and having problems with the grout cracking when the flexible silicone was compressed by random impacts on the tiled surface. I also discouraged people from using it because silicone doesn’t clean up easily with soap and water like Weldbond and thinset mortar do, and novices tended to use whatever silicone caulk they happened to have at home, no matter how old.  They also expected silicone to compensate for using vulnerable backers like plywood, which isn’t reasonable.

The problem is that Weldbond doesn’t ever fully dry and turn clear when sandwiched between larger pieces of glass, and there is a real need for something that can be used for glass-on-glass mosaics, such as gluing stained glass to a window pane.

Now we sell GE Silicone II for glass-on-glass mosaics and for projects such as steel mailboxes where thinset will eventually cause bare steel to oxidize or fall off when the paint on painted steel starts to flake off.

15 thoughts on “Mosaics on Steel Mailbox Using Silicone Adhesive

  1. Nguyen Phuc

    Love this article. I honestly never think of using Silver Sharpie before lol. This method can also be applied to decorating various things. This articles just gave one or two ideas about renovating my bathtub.

    1. Joe Moorman Post author

      For an outdoor mural, I would not use silicone. I would use thinset mortar on a conventional backer such as concrete, masonry, or stone.

  2. Donna Howell

    Good morning!!!! I have almost completed my mosaic mailbox. It is my first experience using the silicone and I am always a messy mess with my creations. This one was no exception. What can I use to remove any unwanted adhesive and clean up the tiles prior to grouting? Any suggestions would be appreciated.

    1. Joe Moorman Post author

      Hi Donna,

      They make silicone caulk removers, but they require some rubbing. You would want to apply via a rag with a gloved hand instead of spraying it on, which could get it between the tiles. There will be some elbow grease required, and a scraping tool might help.

    2. Glenyse Widdison

      I’ve done a lot of mosiacing with silicone. I used to get into such a sticky mess and found the only way to clean up was to use a razor blade and a rag with elbow grease…. a term used in Australia to mean hard rubbing.
      Now I’ve discovered the use of Baby Wipes to wipe fingers and tessera as I go and so mostly avoid the mess.
      By the way, I use (Selleys) Roof and Gutter Silicone for all my mosaics… its produced to withstand outdoor conditions, it has neutral cure and will not desilver mirror. It’s also cheaper as its aimed at the building trade and not at specialised art and craft sector.

  3. Jane

    Mosaic Art Supply: you are saying in one place that metal mesh and cement should be added to a metal substrate for exterior mosaics, yet giving this as an example for outdoor mosaics on metal? She glues the tile directly to the metal mailbox.

    1. Joe Moorman Post author

      Fiberglass mesh should not be used for outdoor and wet mosaics because the adhesive used to attach tiles to the mesh is vulnerable to moisture. As an alternative to mesh, use temporary face mounting on Mosaic Mounting Tape.

      Thinset manufacturers don’t recommend using their product on a metal backer because of the problem of the alkali thinset corroding the metal. A mosaic mailbox isn’t an architectural surface requiring strict adherence to best practices.

      I hope this helps.


    1. Joe Moorman Post author

      I don’t like to mosaic on metal, but I would use the silicone discussed in this article. I would use thinset mortar to grout the mosaic because it is more adhesive than grout and tolerates thermal expansion and contraction of the metal.

  4. Patricia A Edwards

    Hello, i mosaiced my mailbox about 14 years ago and used E6000, it took about 11 years for the tiles to start coming off. now i’m going to do another one using silicone as you suggest, but i was wondering if the best way would be to put the silicone on the tile and then attach to the metal, so when i grout the grout is touching metal , or should i cover the metal with silicone then put the tile on, (a little space at a time so it want dry) then when i grout, all grout will be touching the silicone and not the metal.
    I hope i made the question clear. i just dont know what would be better, grout touching silicone, or grout touching metal?

    1. Joe Moorman Post author

      Bare metal would be a problem, but I suspect that the metal is painted or clear coated and not actually bare.


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