Choosing the right exposed aggregate retarder depends on where the concrete is being placed and when the retarder needs to be applied. Prime Retarder, Prime Hill Assist Retarder and Prime Formgel are designed for different site conditions—they are not three versions of the same application.
This guide explains when to use each product, how it is applied and the key differences to consider before the pour.
Quick Comparison
| Product | Best suited to | When it is applied | Application |
|---|---|---|---|
| Prime Exposure Retarder | Flat slabs and general horizontal or vertical exposed aggregate work | After the concrete has been placed and finished | Sprayed undiluted onto the concrete surface |
| Prime Hill Assist Retarder | Slopes, steep driveways, stairs, inclines and declines | After the concrete has been placed and finished | Sprayed onto the surface; its thicker viscosity helps limit runoff |
| Prime Exposure Formgel | Formed faces, walls, panels, steps and other formwork applications | Applied to the formwork before concrete is poured | Brushed or rolled onto the formwork undiluted |
1. Prime Exposure Retarder: The General-Purpose Choice
Prime Exposure Retarder is a fluorescent green, water-soluble surface retarder used to expose aggregate in freshly placed concrete. It slows the setting of the surface mortar so the top layer can be washed away while the concrete underneath develops strength.
It is the most suitable option for standard exposed aggregate slabs, including driveways, paths, patios and other relatively flat surfaces.
How to Apply Prime Retarder
- Place, screed and finish the concrete as required.
- Trowel out any tool lines and wait until the bleed water has dried. The surface should still contain enough moisture to pit slightly when the retarder is sprayed.
- Apply Prime Retarder undiluted in a uniform coat at approximately 0.2–0.3 litres per square metre.
- Do not trowel the retarder into the concrete. Doing so can trap product below the aggregate and increase the risk of blow-outs during washing.
- In warm or windy conditions, a second coat may be required after the first coat has absorbed—approximately 30 minutes after the first application.
- Once the fluorescent green colour is no longer visible and the retarder has set, protect the slab with builders film or apply Prime Evaploc in accordance with its technical sheet.
- For the most predictable result, keep the surface protected and wash it the following morning. Experienced contractors may undertake same-day washing when conditions allow, but this carries a greater risk of under- or over-exposure.
The current Prime technical sheet recommends a pressure cleaner rated at 3500 PSI or higher for washing exposed aggregate. Wash-off timing will vary with concrete strength, weather and site conditions.
2. Prime Hill Assist Retarder: For Slopes, Stairs and Gradients
Prime Hill Assist Retarder performs the same basic role as Prime Retarder but has a thicker viscosity. This helps it stay where it is applied instead of running down steep or vertical surfaces and collecting in low areas.
Choose Hill Assist for:
- Steep driveways
- Sloped slabs
- Stairs and steps
- Inclines and declines
- Areas where a standard liquid retarder may run or pool
Its fluorescent green colour makes coverage easier to see and helps identify missed areas. The store product specification lists approximate coverage of 70–90 m² per 20L drum, depending on the required exposure, substrate and application conditions.
How to Apply Hill Assist
Prepare and finish the concrete in the same careful manner as a standard surface-retarder application. Apply an even coat over the required area using suitable spray equipment, paying particular attention to edges, changes in slope and areas where runoff could otherwise create uneven exposure.
Because application rate and wash-off timing are affected by concrete mix, strength, temperature, wind and the required finish, always follow the current product label and Prime Exposure instructions for the job.
3. Prime Formgel: For Formwork and Formed Faces
Prime Exposure Formgel is different from the two surface-applied retarders. It is a thick, water-based gel applied directly to formwork before the concrete is poured. When the forms are stripped, the retarded surface mortar can be removed to reveal the aggregate.
Formgel is suited to formed concrete faces, walls, panels, steps, rebates and other areas where the exposed finish is created against formwork.
How to Apply Prime Formgel
- Ensure the formwork is clean and ready for coating.
- Apply Formgel undiluted with a paint brush or roller. It is supplied ready to use and does not need stirring or mixing.
- On porous formwork, apply the first coat approximately one hour before the pour. Allow it to dry, then apply a second coat around 30 minutes before pouring.
- On non-porous formwork such as Formply, one generous coat approximately 30 minutes before the pour is generally sufficient. The required coat will depend on the desired exposure depth.
- Place the concrete carefully so tools do not scrape the Formgel away from the formwork.
- Strip the formwork within the manufacturer's stated window of approximately 6–48 hours, depending on concrete strength and weather.
- Wash the surface immediately after stripping. Assess the surface first, then use a stiff broom or brush with a hose, or a suitable pressure cleaner, to remove the slurry.
Do not dilute Formgel. Rain or excessive moisture can dilute the coating and reduce its effectiveness. The current store specification lists approximate coverage of 50–60 m² per 20L drum and an application temperature range of 5–30°C.
Which Prime Retarder Should You Choose?
- Choose Prime Retarder for general exposed aggregate slabs and relatively flat work.
- Choose Prime Hill Assist when the surface has a noticeable gradient or the retarder needs greater resistance to runoff.
- Choose Prime Formgel when the exposed finish is required on concrete cast against formwork.
If a project includes both a flat slab and a formed vertical face, more than one retarder may be required. Use the product designed for each part of the pour rather than trying to make one application method suit every surface.
Sprayers and Equipment
Uniform coverage is critical when applying Prime Retarder or Hill Assist. Use clean, suitable spray equipment that can deliver a consistent coat across the entire area without gaps or heavy patches.
Browse our concrete sprayers for application options available from Gippsland Concreting Supplies. Clean the sprayer promptly after use in accordance with the retarder and equipment instructions.
Formgel should be applied with a brush or roller rather than sprayed.
Common Mistakes to Avoid
- Using standard retarder on a steep slope where runoff is likely
- Applying Formgel to the concrete surface instead of to the formwork before the pour
- Diluting products that are supplied ready to use
- Leaving missed patches or applying an uneven coat
- Trowelling Prime Retarder into the concrete
- Allowing rain or moisture to dilute Formgel before the pour
- Relying on a fixed wash-off time without considering weather, concrete strength and surface condition
- Waiting after stripping before washing a Formgel-treated surface
Safety and Site Care
Wear suitable gloves, eye protection, impervious footwear and protective clothing. Keep products in their original labelled containers and prevent spills or wash water from entering stormwater drains and waterways. Review the current product label, Technical Data Sheet and Safety Data Sheet before use.
Technical Information
- Prime Retarder Technical Data Sheet
- Prime Retarder Safety Data Sheet
- Prime Hill Assist product information
- Prime Formgel product and application information
Shop the Prime Exposure Retarder Range
Match the product to the surface before the pour for more even coverage and a more predictable exposed aggregate finish.
Contact Gippsland Concreting Supplies if you need help matching the retarder and application equipment to your job.
