Franking Machine Sealing Solution vs Water: 2026 Guide

If your franking machine has a sealing function, the liquid you put in its tank decides how well it works and how long the moistener lasts. This guide explains what franking machine sealing solution actually is, why tap water is a false economy, how envelope choice affects sealing, and how to dose and maintain the tank so envelopes close first time.

What franking machine sealing solution actually is

Sealing solution is a formulated wetting fluid that sits in your franking machine's sealer tank. A brush or sponge draws it up and wipes it across the gummed flap as the envelope passes through, activating the adhesive.

The fluid is not just water with a label on it. It is blended to do three jobs at once: wet the gum evenly, resist the bacterial and fungal growth that thrives in a warm tank of standing liquid, and limit the mineral scale that builds up on brushes, wicks and tank walls.

Most UK mailrooms buy it in 500ml or one-litre bottles. At Franking Machine Warehouse, 500ml bottles of sealing solution start at £8.95 and one-litre bottles from £13.15. Folder inserters use a separate but similar product, so if you run both machines, check you are buying the right version for each.

Why tap water is a false economy

Filling the tank from the kitchen tap is the most common mailroom shortcut, and the one that costs the most over time.

Much of the UK sits in a hard water area, and hard water leaves mineral deposits behind as it evaporates. Those deposits collect on the moistener brush and in the feed channel, stiffening the bristles and restricting flow. A clogged brush cannot lay down an even film, so flaps seal along part of their length and lift along the rest.

The second problem is biological. A tank of plain water held at office temperature is a good home for bacteria and mould. Within a few weeks you get a stale smell that transfers to outgoing post, and a slimy film on the sponge that stops it absorbing properly. Sealing solution is formulated with antibacterial agents precisely to prevent this.

The third problem is surface tension. Plain water beads on gum rather than spreading across it, so flaps seal in patches. Set against a bottle that lasts most offices weeks, an engineer callout for a seized moistener assembly is a poor trade.

Your envelopes matter as much as the fluid

No sealing solution will rescue the wrong envelope. Franking machine sealers only work on gummed flaps, where a dry, inert adhesive is applied to the flap and activated by moisture.

Self-seal and peel-and-seal envelopes are different animals. Self-seal uses a latex cohesive on two strips that bond to each other under pressure, and wetting it does nothing useful. Peel-and-seal has a pressure-sensitive adhesive protected by a release strip, which a machine cannot remove. Run either through a sealer and you get damp, unsealed post.

Gummed also wins on shelf life. Because the adhesive is inert until wetted, gummed envelopes can sit in a stockroom for years. Latex-based self-seal typically has a usable life of around twelve months from manufacture, and humid storage shortens it further, so gummed protects a bulk purchase better.

Flaps also need to be a consistent size and the paper needs enough body to travel the feed deck without buckling. Our range of business envelopes is specified for machine feeding, with a separate folder inserter envelope range for higher volume setups.

Filling, dosing and maintaining the tank

Good practice is straightforward and takes minutes a month.

• Check the dilution on the bottle. Some solutions are ready to use and some are concentrates. Over-diluting removes the protection you paid for.
• Fill to the marked line, not to the brim. An overfilled tank floods the brush, which soaks flaps and causes cockling and jams.
• Do not top up indefinitely. Empty and refresh the tank rather than adding to an old mixture that has concentrated through evaporation.
• Rinse the tank and brush monthly. Warm water and a soft brush will lift most residue. Let everything dry before refitting.
• Replace the brush or sponge when it hardens. A stiff brush will never seal properly, whatever fluid you use.
• Empty the tank before a long shutdown. Leaving fluid in over a two-week close-down is how mailrooms return to a blocked moistener in January.

If your sealer can be switched off in the machine settings, turn it off when you are running franking labels or already-sealed items, so the brush is not running dry.

Troubleshooting envelopes that will not seal

Work through these before calling anyone out. Flaps lifting after a few minutes usually means too little fluid reaching the gum, so check the tank level and then the brush for scale or hardening. Wet, wrinkled envelopes mean the opposite: lower the tank level and check the brush is not saturated.

If only part of the flap seals, the brush is worn unevenly or the envelopes are feeding skewed, so square up the stack and reseat the side guide. If nothing seals at all, confirm the envelopes are genuinely gummed and that the sealer is switched on. And if the post smells stale, the tank has gone off: empty it, rinse thoroughly, and refill with fresh solution.

Frequently Asked Questions

Can I use tap water in my franking machine instead of sealing solution?

You can, but it is not recommended. Tap water leaves limescale that stiffens and clogs the moistener brush, and standing water in a warm tank grows bacteria and mould that cause odours and poor absorption. Sealing solution is formulated with antibacterial agents and wets gummed flaps more evenly, so envelopes seal consistently and the moistener lasts longer.

How long does a bottle of franking machine sealing solution last?

It depends on your mail volume and tank size, but most small and medium mailrooms find a 500ml bottle lasts several weeks to a few months. Evaporation matters as much as usage, so a low-volume office that keeps a full tank may get through a bottle faster than the envelope count alone suggests. The one-litre size is usually cheaper per millilitre.

Will a franking machine seal self-seal envelopes?

No. Franking machine sealers only work on gummed envelopes, where a dry adhesive on the flap is activated by moisture. Self-seal envelopes use a latex cohesive that bonds under pressure, and peel-and-seal envelopes have a release strip a machine cannot remove. If you want your franker to seal automatically, buy gummed envelopes.

Is franking machine sealing solution the same as folder inserter sealing solution?

They are closely related but sold as separate products, and it is worth buying the version made for your machine. Folder inserters have different tank capacities and brush assemblies, so the fluid is supplied and dosed to suit. Check the bottle description against your equipment before ordering.

How often should I clean the sealer tank?

A monthly rinse of the tank and brush with warm water is enough for most mailrooms, plus a full empty and refresh rather than an endless top-up. Always empty the tank before a holiday shutdown or any period of two weeks or more without use, because fluid left standing is the most common cause of a blocked moistener when the office reopens.

The short version

Sealing solution is one of the cheapest consumables in the mailroom and one of the easiest to get wrong. Use the proper fluid rather than tap water, pair it with gummed envelopes, and keep the tank clean and the brush fresh.

You can browse our full range of franking machine sealing solution and machine-ready envelopes at Franking Machine Warehouse.