If you don't know anything or have questions about the prior sealer that was used, see the following tips. Once you have completed the testing, take the conservative approach. Pick the sealer most like the one that might be below the surface to insure good bonding.

• Solvent base penetrating
• Water base penetrating
• Water base coating

Apply some lacquer thinner to a test area.

1. If the old sealer dissolves completely and tends to penetrate into the surface it could be a solvent based penetrating type. Reseal with another solvent based penetrating sealer.
2. If there is little or no effect on the sealer, it could be a silicone (or other unknown type) and must be considered a high risk situation for any resealing. See also water based penetrating sealer below.
3. If the lacquer thinner makes the old sealer tend to come apart and leave a residue of rubbery particles (like pencil eraser particles) it could be a water based coating type. If so, after stripping it off, water drops should readily and uniformly absorb into the surface. Categorize the surface as being less absorbent than unsealed material of the same kind. If in doubt, use a solvent base penetrating type.
4. If there is no reaction or change from the lacquer thinner, it might be a water based penetrating sealer. That means there will be sealer below the surface after stripping and the ability to absorb a new sealer, or bond with a new sealer is a concern.