We never have to worry about evaluating new adhesives in our work because we use conventional mosaic backers like plywood (indoors), concrete backer board, stone slabs and masonry walls. This means we always use Weldbond (a white PVA adhesive for dry indoor mosaics) and thinset mortar for everything else.
But what if you are wanting to put a mosaic on a plastic mannequin or a fiberglass sculpture or some other novel backer?
Well, these backers were made to be light weight, and so it doesn’t make sense to use thinset, which is a type of concrete, and Weldbond does not bond at all to most plastics.
Silicone and Epoxy
Many people report using silicone-based adhesives and epoxies on plastics with good results, but there are several reasons to test whatever adhesive you select on your particular backer before investing time attaching very many tiles.
Reasons to Test Your Adhesive
Plastic Ain’t Plastic
There are many different types of plastics. The information you read online might say the product works well on plastic or bowling balls or whatever, but chances are that whatever plastic you have isn’t exactly the same as what the author used in their project, especially if you are using something found at a thrift store or yard sale and possibly several decades old.
Delamination of Composite Materials
The problem might actually be within the substrate and not the glue per se. For instance, testing might reveal that your plastic is actually a composite material with a thin outer layer that pulls off relatively easily when something is glued to it.
Glues Can Go Bad
What if the problem is with your particular container of glue? Specialty adhesives have shelf lives and expiration dates and can sometimes be ruined by exposure to heat or cold.
The easiest way to test an adhesive is to put a small bead or dot of adhesive in an out-of-the-way place on your backer and try scraping it off a few days later with a paint scraper or putty knife. Remember to use common sense and not damage your sculpture. If the bead of adhesive won’t come off, don’t keep banging on it with the scraper until you come away with a big chunk of the substrate or crack your sculpture. Instead, use sandpaper to sand the adhesive down.
To make your test bead, you will also want to rub the wet adhesive onto the surface to make sure that the bead of adhesive has actually made intimate contact with the surface and isn’t just sitting on top superficially. You don’t have to smear it flat. It can still be a small bead.
I recommend putting two or three beads of adhesive in the test area to make sure that you have at least one that has good contact. If the adhesive fails the test, you want to know it failed because the adhesive isn’t meant for the material, not because you didn’t apply it very well. For the same reason, you will want to make sure that you prepare the surface before testing.
Before applying glue, you need to make sure the surface is clean and free of dust and oils. Most molded plastic surfaces are fairly smooth, so you will also want to lightly scuff it with a medium-grit sandpaper. However, you shouldn’t skip cleaning the surface before sanding it. If there are oils or contaminants on the surface, it is possible for them to migrate down to the freshly exposed material as you are sanding. The sandpaper gets contaminated and then contaminates the new material underneath.
I use dish-washing detergents for cleaning applications like this because they wash away cleanly. Solvents like alcohol might be more effective for lifting grease with less scrubbing, but they also run the risk of being absorbed by the material being cleaned. Probably not, but it’s safer to assume the worst when using a base of unidentified materials.
Which Brand to Use?
Consider brands like Loc-Tite, DAP and Permatex. Loc-Tite makes a variety of adhesive products in addition to epoxies and silicone adhesives, so make sure you don’t buy one of their super glue (cyanoacrylate) products. Cyanoacrylate bonds are strong but tend not to last as long because they are brittle.
Gorilla Glue is a tougher and more impact resistant cyanoacrylate adhesive, and it does have a lot of fans, but we have not used it. I would be interested in learning more about how it ages. If the tiny rubber particles that give Gorilla Glue its toughness were to oxidize over time and dry rot the way rubber does, then I would have serious doubts about using it in art meant to be durable.
Going to the adhesive aisle of a building material store and reading a few labels is a good way to find a few candidates and compare them side by side based on manufacturer recommendations. Usually the packaging will have a list of materials the adhesive will work on. Online shopping might be less useful in this regard because the product descriptions are often fairly brief, but the flip side is you can read product reviews.
HOWEVER, no matter how much of that type information you gather, make sure you test the adhesive on your particular plastic/fiberglass backer before committing time on the actual tiling.