Guide
How to choose a protective coating
Most people compare two products on a single number. It is almost always the wrong number. Here is how to judge one, and what a coating can and cannot do.
Solids content: what it actually measures
Solids content is the percentage of material left once the carrier, water or solvent, has evaporated. It is a real measurement. It is simply not an overall grade, and for this category of product it is often the wrong question.
Where it matters
For a film-forming product, a paint, an elastomeric membrane, a sealer that builds a layer on the surface, solids content determines dry film thickness per coat. More solids, more film. There, comparing two products on that number makes sense, because both are doing the same thing: building thickness.
Where it does not
For a penetrating product, a silane/siloxane water repellent, the job is not to build thickness but to migrate into the pore structure and bond there. What decides the result is molecule size, active chemistry and depth of penetration. A high solids number tells you none of that.
What the number does not say
A percentage does not say what the solids are made of. Inert fillers inflate the figure without adding performance. Two products with identical solids can have almost nothing in common.
When a high number becomes a defect
On a surface that needs to breathe, high solids achieved with film-formers create a vapour-tight layer. Moisture coming from inside the material stays trapped behind it. That is where blistering and peeling come from, usually a year or two later.
In short: compare products that do the same thing first. Comparing the solids content of a penetrant against a film-former is comparing two answers to two different questions.
The main families, and what each one costs you
No brands named: these are the trade-offs inherent to product TYPES, not complaints about anyone. Each has its use, the problem is using it in the wrong place.
Film-forming and elastomeric coatings
They build a layer on the surface, which gives an immediately visible result and genuine waterproofing. In exchange: they change the appearance, they can trap substrate moisture, and a film that lets go does not patch well, it usually has to come off before you start again.
Solvent-based repellents
Good penetration and good durability. The trade-off is elsewhere: volatile organic compounds, odour, handling and ventilation constraints, and an aggressiveness that does not suit every substrate or every sealant already in place.
Pure silane
Small molecule, deep penetration. Excellent on dense concrete. But it is more volatile: some of it evaporates before reacting, and protection right at the surface wears out sooner. That is why it is usually blended with siloxane rather than used alone.
Acrylic or wax-type sealers
Cheap and spectacular for the first few months: water beads immediately. They break down quickly under UV, though, and need frequent reapplication. On a roof in full sun, that is the most expensive trade-off over time.
The "one coat" promises
A stated coverage figure almost always assumes an ideal substrate. Smooth and barely porous. Aged shingles or open masonry drink far more. Coverage that does not state the number of coats or the surface condition is not comparable.
What a coating like ours should do
- Repel water. Clean, immediate beading, and measurable (the RILEM tube exists for exactly this).
- Let the material breathe. Vapour has to get out, otherwise you move the problem instead of solving it.
- Resist UV, because it is the sun, not the water, that destroys most long-term protection.
- Follow microcracks without breaking.
- Extend the life of a surface that is still sound.
What it should not claim to do
- Repair a leaking roof. A coating is not a repair, a leak is fixed first, not with a product.
- Replace a roof at the end of its life. Protecting a finished surface does not bring it back.
- Deal with moss and algae. Those are cleaned off before application; coating over them seals them in and the problem keeps going underneath.
- Change the structure or load capacity of anything.
- Last the same everywhere. Real exposure, full sun, freeze, standing water, changes the outcome.
The questions to ask about any product
If a supplier cannot answer these, solids content will not save you.
- Does it penetrate or does it form a film? The two are not comparable.
- Does the surface stay vapour-permeable?
- On exactly which substrates, and which are excluded?
- What coverage, at how many coats, on what surface condition?
- What does the warranty say, and what voids it? Self-application is generally covered by nothing.
- What has to be done before applying? Preparation decides most failures.
Questions
Does a higher solids content mean a better product?
No. Solids content measures the material left after the carrier evaporates. It is useful for comparing film-forming products to each other, because it determines film thickness. For a penetrating water repellent, what matters is the active chemistry and the penetration, not thickness, and a high number can even signal a film that will stop the material from breathing.
What is the difference between a penetrating and a film-forming product?
A penetrant migrates into the pore structure of the material and bonds there, leaving the surface able to breathe. A film-former builds a layer on top. The film-former gives immediate waterproofing but can trap substrate moisture and eventually blister or peel.
Why does breathability matter?
Because moisture inside the material has to get out. A vapour-tight layer holds it behind the coating, which causes blistering, peeling and, in a freeze climate, damage to the substrate itself.
Can a coating repair a leaking roof?
No. A protective coating extends the life of a surface in good condition. A leak is repaired before application, and a roof at the end of its life has to be replaced, no product changes that.
Does moss have to be removed before applying?
Yes. Moss and algae are cleaned off before application. Coating over them seals them under the product and the problem keeps progressing underneath.