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Professional Guide

Best Rapid Set Post Mix for Fencing: 2025 Expert Review

DJ

DR Joinery

Expert Joiner & Timber Specialist

Published 16 December 2025

There is nothing more frustrating than looking out into the garden after a gale to see a fence leaning drunkenly because the ground has softened. In my years working on properties in York, I've seen exactly what happens when fence posts aren't set correctly or when inferior concrete is used.

Using the right rapid set post mix makes the difference between a fence that lasts for fifteen years and one that starts wobbling in six months. It isn't just about dumping a bag in a hole; it's about choosing a mix that cures hard, resists moisture, and locks the timber (or concrete spur) firmly in place.

In this review, I'm looking at three specific options: a budget-friendly repair mix for small patches, a heavy-duty option for new installations, and a reliable alternative for quick, hassle-free setting.

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Specs vs. The Real World

The Spec: "Set in under 10 minutes"

The Theory

"Marketing materials claim the post will be rock solid in minutes, allowing you to hang panels immediately."

The Reality

In typical UK weather (especially if it's damp or cold), 'set' just means it supports its own weight. I always advise leaving it for at least an hour before applying any force, like hammering in nails or screws.

The Spec: "No Mixing Required"

The Theory

"Just pour the dry mix into water in the hole."

The Reality

This works, but you often get dry pockets of powder that never cure. A sturdy stick or metal rod should be used to 'joggle' the mix in the hole to release air bubbles and ensure water saturation.

My Top 3 Recommendations

Three options to cover different needs and budgets

Everbuild Jetcem Rapid Set Cement – 3kg Bag

Budget Pick
Everbuild Jetcem Rapid Set Cement – 3kg Bag

Everbuild

This is distinctly different from the full-sized bags below. At 3kg, you will not be setting a full fence post with this. However, it is an essential product for the 'budget' category because it saves you buying a 20kg bag just to repair a crack or patch a loose spur. It sets incredibly fast (30 mins) and is ideal for maintenance.

Price£7.91 (at time of writing)
Weight3kg
Set TimeApprox. 30 minutes
Best UseRepairs, patching, filling cracks
The Good
  • Perfect size for small repairs without waste
  • Sets extremely hard very quickly
  • Ideal for pointing around the base of an existing post
The Bad
  • Too small for setting a new fence post (need approx 20kg)
  • Grey colour might not match weathered concrete perfectly

Installer Tip:

If you have a concrete spur that has cracked slightly or needs pointing up to stop water pooling at the timber base, use this. Trowel it to a slope away from the timber to encourage water runoff.

Priced at £7.91 at the time of writing

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Fast Set Post Fix Concrete - 20kg Bag

Quality Option
Fast Set Post Fix Concrete - 20kg Bag

Generic

This is a heavyweight, standard 20kg bag designed for the main job: setting fence posts, gate posts, or decking uprights. Whilst the price is higher than standard trade yard rates, the convenience of delivery for a 20kg product is significant. It offers a strong, durable hold suitable for professional-grade installations.

Price£17.44 (at time of writing)
Weight20kg
Set TimeUnder 10 minutes
ApplicationPour and add water
The Good
  • Rapid setting allows for quicker job completion
  • Strong hold suitable for tall fencing and gates
  • No pre-mixing required
The Bad
  • Heavy to manoeuvre if you order multiple bags
  • Price per bag is higher than bulk-buying at a merchant

Installer Tip:

For a standard 100x100mm (4 inch) fence post, one bag is usually the minimum. If you are in Soft York clay or loose soil, dig a slightly wider hole and use 1.5 to 2 bags per post for a foundation that won't move.

Priced at £17.44 at the time of writing

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Laeto Fast Setting Concrete – 20kg Quick-Set Post Mix

Problem-Solving
Laeto Fast Setting Concrete – 20kg Quick-Set Post Mix

Laeto

The 'problem' this product solves is twofold: speed and storage. The waterproof packaging is a genuine benefit in the UK; standard paper cement bags ruin instantly if left on damp grass. This Laeto mix is a reliable 'Postcrete alternative' that sets in under 10 minutes, making it ideal for jobs where time is of the essence.

Price£15.99 (at time of writing)
Weight20kg
PackagingWaterproof bag
Set Time< 10 minutes
The Good
  • Waterproof bag prevents wastage during storage
  • Slightly cheaper than the other 20kg option reviewed
  • Reliable set time for quick progress
The Bad
  • Still requires manual handling of 20kg weight
  • Ensure you have enough water on site before starting

Installer Tip:

Dig the hole, fill 1/3 with water, then pour the mix in steadily. This ensures the water penetrates from the bottom up. If you pour powder first and water second, you risk a dry, powdery bottom that offers no structural strength.

Priced at £15.99 at the time of writing

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Frequently Asked Questions

How many bags of post mix do I need per fence post?
For a standard 2.4m post (buried 600mm deep) in a hole that is roughly 200-250mm wide, you generally need 1.5 bags of 20kg post mix. Using just one bag is often possible in very firm ground, but skimping on concrete is a false economy. In soft ground, always budget for 2 bags per post.
Can I use rapid set mix in the rain?
Yes, you can use it in the rain as it requires water to activate anyway. However, verify the hole hasn't collapsed or filled with mud slurry first. If the hole is full of muddy water, pump it out before adding the mix, otherwise, the concrete will be diluted and weak.
Is post mix better than mixing sand and cement?
For fencing, yes. Post mix is formulated to set very quickly (5-10 minutes), holding the post upright without the need for extensive bracing. Traditional sand and cement is cheaper but takes 24 hours to go off, meaning you have to prop every single post and wait a day before attaching panels.
Why shouldn't I use the 3kg bag for a fence post?
A 3kg bag (like the Everbuild Jetcem reviewed) simply doesn't provide enough mass or ballast to counter the leverage of a wind-blown fence panel. It is designed for patching and repairs. A fence post relies on the heavy 'bulb' of concrete at the base to stay upright.
Should I concrete the timber post directly?
Ideally, no. Even treated timber will eventually rot at ground level (the collar). Professional practice in York often involves using a 'concrete spur' or a metal post shoe set in concrete, with the timber bolted to that above ground level. If you must concrete the timber directly, slope the concrete away from the post to shed water.

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