When you need primer (and when it gets in the way)

It is not a “just in case”

Primer is a chemical bridge between substrate and sealant. It does not make every joint stronger. On clean, degreased glazed tile, a neutral sanitary — OTTOSEAL S100 — usually grabs without it. On a mineral that soaks, or a meeting that lives under water, the system sometimes wants primer. It depends on the pair of substrate plus cartridge chemistry, not on a bathroom recipe.

Putting primer “for extra strength” on a dirty or wet flank is bonding the bridge to dust. The sealant leaves with the dirt.

When yes — and when not

Yes, often: porous concrete or stone, untreated timber, meetings with heavy mechanical load or standing moisture (places where water does not leave).

No, often: smooth glass, sanitary ware and glazed tile properly cleaned, non-critical interiors.

Not any random primer. Stone primer is not glass primer; acetoxy is not the same as neutral. The wrong one gets in the way.

Degreasing is not priming. The cleaner takes off grease and film. Primer changes the chemical face of the substrate. First one, then — if needed — the other, with the flash-off that primer asks for.

How I do it

1. Cut out the old. Vacuum the dust.
2. Degrease non-porous faces in one direction, clean cloth.
3. See whether the substrate soaks. In doubt, do not improvise a random tin from the shop: either the product’s system, or leave it.
4. If there is primer, wait until it is dry to the touch as it should be before you gun silicone.
5. Backer rod if the void is deep. Then the bead.

Mistakes I see

Primer on wet or dusty faces. Skipping flash-off. Thinking a bathroom refit “always takes primer”: on ordinary glazed tile, almost never, if the technical clean is done.

Exceptions

Stone and standing water: pick stone chemistry and the primer that belongs with it, or skip primer and change product. Do not mix systems.