It is essential to the design structure of an outdoor mosaic to weatherproof the base so your work of art lasts a long time. A cardboard tube can serve as the base for a mosaic column. The ideal way to protect the tube from harsh weather is to waterproof it. When the cement and tile are added during the mosaic creation process, it provides an additional layer of protection.
Buy a container of elastomeric roof coating. The product is fungus and mould resistant as well as waterproof. Check with online retailers if you can't find the product in your local home improvement centre.
Wipe the cardboard tube off with a dry cloth to remove any cardboard shavings or debris that may interfere with the roof coating adhesion.
Place a dust sheet or newspaper coating on the floor your workspace. Ideally you should do this outside or in a very well-ventilated area to avoid fume build-up from the roof coating.
Apply the elastomeric roof coating to the interior of the cardboard tube by attaching a narrow paint roller to a stick that's wide enough to fit through the tube hole.
Use a small paint roller or brush to apply the elastomeric roof coating to the exterior of the cardboard tube. Cover as thoroughly as possible.
Allow time for the coating to dry based on specific instructions for your product before applying the mosaic cement and tiles.
Dave Friant, the author of "The Cardboard Boat Book" recommends elastomeric roof coating as the best way to waterproof cardboard.