Passive Purple Application FAQ: Coverage, Coats and Drying Times

Passive Purple is applied in two coats by airless spray, roller or brush, over a suitably primed substrate, and can be plastered or rendered once the membrane has cured. Those are the essentials — but the questions we hear every day from installers deserve fuller answers. This post gathers the most common application questions about Passive Purple in one place: how it goes on, why it is always a two-coat system, which primer to use where, what you can apply over the cured membrane, and how fast a spray crew can realistically move. For exact coverage rates and drying times, always work from the product TDS and our application and coverage rates page (linked at the end of this post) — those figures are substrate- and site-dependent, so we keep them in one maintained location rather than repeating them here.

How is Passive Purple applied?

Passive Purple is a liquid-applied airtight vapour control membrane, so application feels familiar to anyone who has used heavy-bodied coatings. You have three options:

• Airless spray — the fastest method for whole-house airtightness, and the standard choice for site crews working to programme.
• Roller — well suited to smaller areas and rooms where masking for spray is not worthwhile.
• Brush — for detailing: corners, penetrations, frames and junctions. Passive Purple Brush is formulated specifically for brush and roller application.

Typical sequence: prepare and prime the substrate, apply the first coat, detail penetrations and junctions while access is easy, then apply the second coat to reach the specified dry film thickness.

Why two coats — always

Passive Purple is a two-coat system. The membrane's certified airtightness performance depends on achieving the required dry film thickness, and that is reached with two full coats — never one. The first coat seals and evens out the substrate's suction; the second builds the membrane to thickness and closes any pinholes left by the first pass. Skipping the second coat is the single most common cause of disappointing airtightness test results with any liquid membrane, because a thin membrane is an incomplete membrane. Check thickness as you go and follow the TDS.

Which primer do I need?

Most substrates benefit from priming, and some require it. The rule of thumb from our primers range:

• Lime Prime — for lime plasters and lime-based substrates, common in older and heritage buildings. Lime surfaces are alkaline and often friable, so this dedicated primer is the correct base.
• Primer IM — the general-purpose choice for porous or dusty substrates such as blockwork, brick, render and concrete, stabilising the surface and controlling suction before the first coat.
• Supergrip — applied over the cured Passive Purple membrane before plastering, giving the plaster a mechanical key onto the membrane surface.

If you are unsure which primer your substrate needs, ask our technical team before you start — the right primer is much cheaper than an adhesion failure.

Can I plaster or render over it?

Yes. Once Passive Purple has fully cured, it can be plastered, rendered or boarded over as part of the normal finishing sequence. For plaster directly onto the membrane, apply Supergrip first to provide the key. The membrane then sits protected behind the finish for the life of the building, exactly where an airtight vapour control layer should be. Cure times vary with site temperature and humidity, so confirm the membrane is fully cured per the TDS before finishing.

How fast can a crew move?

Speed is one of the main reasons contractors switch from sheet VCLs to liquid membranes. With an airless spray set-up, application rates of up to 300 m² per day are achievable, with detailing done by brush as the crew works. There are no laps to tape, no boards to cut around, and the membrane bonds directly to the substrate — so the programme saving on a whole house or apartment block is substantial. Browse the full Passive Purple range to match pack sizes to your project.

Storage and shelf life

Store Passive Purple in a cool, dry place, away from direct sunlight and frost, with containers kept sealed when not in use. In original unopened packaging the product has a shelf life of 24 months from the date of manufacture. On site, protect pails from freezing — it is a water-based product.

Frequently asked questions

What coverage will I get per pail?

Coverage depends on substrate porosity, application method and the dry film thickness specified for your project, so we do not quote a single figure here. Use the product TDS and the application and coverage rates page linked above to calculate quantities for your job.

How long does Passive Purple take to dry between coats?

Drying and cure times depend on temperature, humidity and ventilation on your site. The TDS gives the figures to work from — as a rule, confirm the first coat is dry before the second coat goes on, and confirm full cure before plastering.

Do I really need two coats if the first looks well covered?

Yes. Visual coverage is not the same as dry film thickness. Passive Purple is certified as a two-coat system, and both coats are needed to reach the membrane thickness that delivers the airtight performance.

Can I spray Passive Purple with ordinary decorating equipment?

No — it is a heavy-bodied membrane and needs an airless spray unit. For small areas, roller and brush application with Passive Purple Brush avoids the need for spray kit altogether.