How can I protect my slate floor from essential oils?
If the cheap sealant was water based then you can top that with another water based sealant, but I don't think that would solve the problem. The essential oils are eating away the water based coating. You will need to chemically strip the old coating then coat with a penetrating sealant that says it protects against oils.

In general, putting a solvent-based sealer on top of a water-based sealer is a bad idea. The solvents can eat or soften the existing water-based sealer, destroying both coatings. Think of it along the lines of oil and water; they do not mix.
If you do want to switch to a solvent-based sealer, you will need to chemically strip off the existing water-based sealer before applying the solvent-based product.
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