Sculpt-or-Coat®1 and Mod Podge®2, used alone or more usually in combination, are the solution. These products are similar to white glue. They are both water soluble and dry completely clear. They can be thinned with water if necessary. The main difference between the two products that I have found is that Sculpt-or-Coat® has a thicker consistency and retains brush strokes when it dries. Even when thinned with water it still retains a textural finish when dry. However, it tends to produce a more rigid finish. Mod Podge® on the other hand is thinner and self-leveling as it dries. This makes for a very smooth surface. By using these two products together you can have the best properties of both.
1 Sculpt-or-Coat® is a trade mark of Sculptural Arts Coating, Inc.
2 Mod Podge® is a trade mark of Plaid Enterprises, Inc., Norcross, GA 30019