Silicone v STP Technology In Roof Coatings

Best Practice Guide

Summary

On steel roof substrates, “moisture tolerance” refers to environmental resistance after curing, not surface condition at the time of application. Steel cannot be meaningfully “damp”; it is either wet or dry. Free surface moisture prevents reliable adhesion at the steel–coating interface and increases the risk of inter-coat failure at laps and day joints. Hybrid SPF / STP systems are particularly sensitive to recoat timing due to rapid skinning and low surface energy once cured. Traditional silicone systems, when applied to dry substrates, provide more predictable inter-coat compatibility and joint integrity on metal roofs.

Key Terms

moisture tolerance – what is moisture tolerance referring to?
dry substrate adhesion – Why this matters more than you think
hybrid polymer skinning – The process of how STP systems cure and what can affect it
inter-coat adhesion – the major issue that you won’t see in marketing materials

“Moisture-Tolerant” Coatings on Metal Roofs: Why the Distinction Matters

In recent years, some cut edge corrosion treatments and roof coating systems—particularly SPF / STP hybrid technologies—have been promoted as “moisture tolerant” or “capable of application in damp conditions.” While this language is attractive operationally, it risks obscuring an important technical reality:

Steel does not become ‘damp’ in the way porous substrates do. It is either wet or dry.

This distinction has direct implications for adhesion, joint integrity, and long-term defect arrest, something that marketing literature will not explain. At Liquasil, because we are surveyor-led, we want professional specifiers and contractors understand the difference between marketing and technical reality.

Steel vs porous substrates: a fundamental difference

Porous materials such as asbestos cement or concrete can retain internal moisture while presenting a visually dry surface. Steel cannot.

On metal, moisture exists only as free surface water or condensation
Any moisture present acts as a physical barrier at the coating–steel interface
That interface determines long-term performance, not the cured coating film alone

For this reason, surface condition at the moment of application is critical on steel roofs.

What “moisture tolerance” actually means in practice

So-called moisture tolerance in hybrid systems generally refers to:

resistance to high humidity or condensation during curing
the ability to recover after brief weather interruptions
tolerance of residual dampness following washing, not standing water

It does not mean that adhesion has been validated to visibly wet steel, nor that surface water can be ignored without consequence.

Independent certification, including by the British Board of Agrément, assesses durability of the cured system when installed in accordance with instructions. It does not endorse wet application as best practice for steel substrates.

The day-joint problem with SPF / STP hybrids

Contractor feedback consistently highlights one recurring issue with SPF / STP hybrid top coats:

limited recoat windows
poor self-adhesion once curing has commenced
sensitivity at day joints and overlaps

These products cure by moisture-triggered cross-linking. Once cured, they become chemically inert and low-energy, meaning:

fresh material cannot chemically fuse with cured material
adhesion at overlaps relies on mechanical key only
missed recoat windows lead to peeling or delamination at joints

Ironically high humidity, the condition often cited as an advantage with STP based products actually accelerates skinning and reduces the margin for error at laps.

Why traditional silicone behaves differently

Traditional silicone systems, including those used by Liquasil Ltd and other legacy systems take a different engineering position:

dry substrate specified as standard
moisture tolerance treated as contingency, not method
strong inter-coat compatibility and day-joint forgiveness

Silicones do not rely on moisture to cure in the same way hybrids do, and they remain more receptive to subsequent coats, reducing the risk of self-rejection at overlaps.

The trade-off is well understood:

stricter weather windows
slower installation
legacy systems can suffer more from environmental dirt pick-up, though Liquasil’s systems are designed for this to removed easily with washing

However, from a defect-arrest and risk-management perspective, silicone remains the more predictable option on steel.

The professional conclusion

For metal roof substrates:

“Moisture tolerant” describes environmental resistance, not interface control
Adhesion failures originate at the steel interface and at joints, not in the field of the coating
Dry application is not conservatism, it is defect management

Silicone systems remain better suited where long-term joint integrity, recoat reliability, and dilapidations defensibility outweigh short-term programme convenience.

Technical Rationale vs Product Application

Liquasil uses it’s own purpose-designed recipes to create silicone coatings that are robust and long-lasting, backed up with formal BBA Approval for our metal roof coating, Metalseal and Flexlap cut edge corrosion treatment.