simple gauge made from straight-edged board

How to Build Up Areas and Fill Holes in Mosaic Backers

For dry indoor mosaics, areas can be built up to support thinner tile next to thicker tile by mixing sand or sawdust to Weldbond adhesive to create a heavy paste. You can also fill holes in mosaic backers using this method. Sand is best for minimizing the contraction that happens as the glue dries, but sawdust can be used when weight is an issue.

For outdoor and wet mosaics, thinset mortar can be mixed with clean pea gravel to make a concrete. The pea gravel must be washed clean and dried to avoid contaminating the mortar, and we recommend screening the pea gravel and using only small pieces. Note that you cannot use sand because too much sand will overwhelm the mortar and make it crumbly.

Case Study: Stained Glass Mosaic Coffee Table

Stained Glass Mosaic Coffee Table
Stained Glass Mosaic Coffee Table by artist Holly LaBarre.

Recently our customer Holly LaBarre emailed us asking how to build up areas between wooden slats of a mosaic coffee table she was making. The design concept for the coffee table was to make it look like it was made from pallet wood or wooden planks and put stained glass mosaic between the planks. The problem was that the planks are significantly thicker than the stained glass, but Holly wanted the stained glass to flush with the planks so that the coffee table had a flat surface.

The solution was to put a piece of 1/2-inch plywood on the underside of the table top and then build up the area to be mosaiced using a paste of Weldbond and sand.

Mosaic backer built up
Mosaic backer built up. This photo shows one trough with an initial filling of paste. The other two troughs are still empty and show the plywood backer underneath the table top.

The catch is how to build up the area to the right height so that when the stained glass is applied, it isn’t too high or low. To figure this out, Holly made a simple gauge from a straight edged piece of wood that rests on the surrounding planks and has a small piece of stained glass glued to its bottom edge.

simple gauge made from straight-edged board
A simple gauge made from a straight-edged board and a piece of stained glass glued to the bottom shows how high the area needs to be built up. This photo shows that more paste needs to be added, probably 1/2 inch more.






4 responses to “How to Build Up Areas and Fill Holes in Mosaic Backers”

  1. Katie Avatar

    How exactly did you make the paste? Did you literally buy sand and mixed it with the Welbond or is there a premade mixture? What would the ratio of sand to welbond be? Also did she let this mixture dry completely before laying the tiles down?

    1. Joe Moorman Avatar
      Joe Moorman

      Hi Katie,

      The ratio of Weldbond to added solids isn’t exact. I would start with one-to-one and mix in more if it still looks watery. Building material stores sell sand, but you might but you might be able to find some for free. I would allow it to dry completely and form a firm surface before tiling on it so that any contraction can take place first.

      I hope this helps!

  2. Edward J Metz Avatar
    Edward J Metz

    Could you use thinset as a base of this kind?

    1. Joe Moorman Avatar
      Joe Moorman

      Thinset mortar “thins” or contracts some when it hardens due to the polymerization of the ethyl acetate. When I need to use thinset mortar as a filler or concrete, I always mix in extra fine pea gravel.

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