STEP ONE – Substrate Selection & Preparation
The first step in making your garden mosaic is to select the best base or substrate on which to place the mosaic.
Q: Is the substrate waterproof & weather-resistant ?
The combination of moisture and low temperature is a major problem for garden mosaics.
Beware of freeze-thaw cycles. Bring your mosaics indoors when temperatures drop to freezing, and keep them in storage until freezes are over in the spring.
It’s not just the water; it’s the combination of water seeping into the cracks and crevices followed by freezing temperatures that can ruin all of your hard work.
Suggested substrates:
- Pre-made concrete stepping stones, bricks, or pavers
- Natural stone
- Stone tiles
- Flagstones
- Concrete
- Terracotta
How do I know if it is waterproof?
Do a test for porosity:
- Drop a small amount of water on the surface of the item.
- If the water is readily absorbed, the surface is porous.
- If the water beads on the surface, it is not porous.
(Terracotta, for example, is quite porous and will need to be sealed. For terracotta you may want to do two coats of waterproofing. Test when it is dry to see if water beads up.)
Waterproof any porous surface.
Suggested waterproofing methods:
- Thompson’s Waterseal
- Weldbond or other PVA glue Method
Spray or paint Thompson’s Waterseal on per the instructions. You can buy a spray can at most hardware stores (Home Depot, Ace, etc.) that will handle several small projects. One coat should do it, but repeat the porosity test when it is dry to confirm. The fumes are notable; this should be used outdoors. (After you spray, you can bring the item into a garage or outdoor shed until it dries, so that it does not get wet prematurely. Because of the fumes, I would not leave it in the house, or around pets or people until it is completely dry.)
PVA (Polyvinyl Acetate glue) glue is the white glue you are used to. It is also called: wood glue, carpenter’s glue, white glue, school glue. Weldbond is an especially good version of PVA glue. It is non-toxic and does not have any fumes, so it is safe to use in the house and around children and pets. Weldbond is available at some hardware stores (Ace) and on the internet. To waterproof the substrate with Weldbond or other PVA glue: mix 5 parts water to 1 part glue, paint it on, and allow to dry. It should dry in about one hour.
Note: Many people use PVA glue as the adhesive for indoor mosaics, but it should not be used for outdoor mosaics. A solution of 3 -10 parts of water to one part PVA glue can also be used to prime a surface before applying mosaic.
This post is part of a series on Mosaics on the Rocks:
