Priming a floor and sealing the corners with acrylic

Primer firms up the surface of the subfloor, binds the last traces of dust and evens out how much water the surface absorbs. Without it, a dry concrete slab or screed pulls the water out of levelling compound or adhesive. The material then cures poorly, cracks or flakes off. Sealing the corners solves a different problem: gaps around the edges of the floor.

Why would the corners have gaps at all? In older houses and flats the plaster often stops short of the floor, hidden behind the skirting board. When the old floor covering and skirting come off, a chunk of plaster or screed often comes with them and leaves a gap under the wall. Around central heating pipes there is usually a hole in the floor that nobody ever filled. Dust gets through these spots, and so does self-levelling compound. It flows like thick cream and finds every opening.

You need this combined item mainly when:

• self-levelling compound will be poured and the edges or pipe penetrations are not tight,
• the floor will be coated (a concrete floor in a garage, cellar or workshop) and you want a clean, closed joint where it meets the wall,
• there are hairline cracks or gaps in the corners that shed dust and would later show through a thin floor covering,
• it is a renovation where removing the old floor left damaged corners and wall bases,
• there is another flat or room below you and you do not want the compound to run through gaps into your neighbour's ceiling.

The price list also has a separate item for priming the subfloor without sealing the corners. Both are measured per m², but the scope of work is different. With this variant, the worker goes round the whole perimeter of the room and every pipe penetration after priming, cleans out the gaps, fills them where needed and seals them. In an ordinary bedroom that means several extra metres of joint. In a flat with lots of alcoves, chimney breasts and pipes, it means far more.

• Priming alone is enough in a new build with fresh screed, where a perimeter edge strip runs round the room and the corners are intact. It is also enough when a floating floor goes down and the skirting board will cover the gap at the wall anyway.
• Priming with acrylic corner sealing suits renovations, pours of levelling compound onto uneven or damaged edges, floor coatings, and anywhere the floor-to-wall joint is visible or leaky.
• Time: with priming alone, the floor area matters most. With this variant, the length of the perimeter and the number of penetrations matter too. A small bathroom full of corners can take longer than a large empty living room.
• Materials: besides primer, you need acrylic sealant, backer rod for deep gaps and sometimes rapid-setting mortar for larger holes.
• Risk: with priming alone, the main risk is choosing the wrong primer. Here a second risk comes in: sealing something that must stay flexible or free, such as the movement gap of a floating screed.

Not sure which variant to order? Look at the corners once the skirting is off. If you see a continuous gap, crumbled plaster or holes round the pipes, this item is the likely choice.

Primer does not work on dust, grease or old adhesive residue. Most problems with levelling compounds come from priming a subfloor that was never properly prepared. Before the crew arrives, these should be done:

• old floor covering, skirting boards, adhesive residue and loose pieces of screed removed,
• plastering finished and dry, at least on the lower part of the walls,
• services in the floor finished and heating pipes in their final position, so nobody drills or chases after the sealing,
• the subfloor dry; with new screed, moisture is usually measured before levelling and the screed manufacturer sets the drying time,
• the room usually above +5 °C, with no direct sunlight or draughts that would dry the primer too quickly,
• the room empty of furniture, with access to power and water.

The crew also needs to know what goes on the floor next. One primer suits self-levelling compound, another suits tile adhesive and another goes under an epoxy coating. Tell the tradesperson in advance, ideally with the name of the system that will be used.

Assessing the subfloor

An experienced floor layer first scratches the surface with a nail or a sharp tool to see whether it crumbles. They check whether it is a cement screed, a calcium sulphate (anhydrite) screed or old concrete, and whether it is a non-absorbent surface such as old tiles. A simple absorbency test helps: drip some water on the floor. If it soaks in straight away, the surface is highly absorbent. If it stays as a bead, it absorbs little or nothing. This decides the type of primer and how much to dilute it.

Cleaning and vacuuming

The floor is swept and then thoroughly vacuumed with an industrial vacuum, including corners and gaps. Dust left in a gap is the most common reason sealant falls out later. Greasy spots are degreased, and loose bits in the corners are scraped out.

First coat of primer

Primer goes on with a roller, a wide brush or, on large areas, a sprayer. It is spread evenly, without puddles, right up to the walls and into the corners. On highly absorbent surfaces, manufacturers often recommend a diluted first coat and a second coat that is less diluted or neat. Puddles must be spread out. Otherwise a glossy film forms on the surface and the levelling compound bonds to it less well.

Preparing and filling the gaps

Once the primer is dry, the gaps along the walls and around penetrations are checked again. Larger holes where sealant would just drop through are filled first with mortar or a rapid-setting compound. Deep gaps get a foam backer rod, so the sealant does not have to fill the full depth. The sealant bead should be a sensible thickness, not a whole centimetre deep.

Sealing the corners with acrylic

Acrylic sealant is pressed from a gun into the joint between floor and wall and around the pipes. It is then smoothed with a damp spatula or a wet finger, so it presses firmly against both sides. Excess is wiped off straight away with a damp cloth, before the sealant starts to skin. Before a pour of levelling compound, the focus is on spots where the compound could escape. This does not replace the perimeter edge strip, which is fitted separately.

Second coat of primer and drying times

If the system requires it, a second coat of primer goes on once the sealant has set. Primers for absorbent surfaces are usually dry within a few hours according to the manufacturer, but some systems ask for a longer wait, for example until the next day. Acrylic usually skins within roughly half an hour, but it cures gradually and a thicker bead needs more time. An ordinary room is usually finished in a day. The next trade starts according to the primer and sealant manufacturers' data.

Primers

• Deep-penetrating primer for absorbent surfaces is the usual choice under levelling compounds and adhesives on cement screed and concrete. It soaks into the surface and strengthens it.
• Primer for calcium sulphate (anhydrite) screeds is made for gypsum-based subfloors, which are sensitive to moisture. Under cement-based levelling compounds and adhesives on anhydrite, the right primer is especially important, otherwise the layers may not hold together.
• Bonding primer for non-absorbent surfaces is used on old tiles, terrazzo or smooth concrete. It is usually thicker, often with sand in it, and leaves a rough surface the compound can grip.
• Epoxy primer goes under epoxy and polyurethane coatings, or onto damper subfloors where the manufacturer's system allows it. Quartz sand is often scattered into the fresh coat.

The golden rule: the primer should come from the same system as the compound or adhesive that follows. Mixing manufacturers often works, but if something fails, the manufacturer may not look kindly on a warranty claim.

Corner sealants

• Acrylic sealant (often sold as decorator's caulk) can be painted over, smooths easily, has limited flexibility and suits dry interiors. It works where the joint will later be painted, covered with compound or hidden behind skirting.
• Silicone is for wet areas and joints that move more. It cannot be painted, and neither compound nor adhesive sticks to it, so it never goes under further layers.
• Polyurethane sealant is more flexible and tougher. It is used with garage floor coatings and on joints with more movement.
• Perimeter edge strip is not a sealant but a foam strip round the edge of the room. With floating screeds and levelling layers it separates the floor from the wall, and sealant should not replace it.

In bathrooms and showers, the corners under waterproofing are treated with waterproofing tape, not acrylic. Acrylic does not belong in areas that stay wet.

The work is simple and quick in an empty, square room with a sound screed and straight walls. The sealant can be run along the corners in one smooth pass, and primer goes on with a roller over a large open area.

More time and material go into:

• an irregular layout with alcoves, chimney breasts, steps and door frames,
• many pipe penetrations, typical of older heating systems with pipes coming up through the floor to every radiator,
• wide, deep gaps along the walls that must be filled first,
• a crumbly or dusty subfloor that needs more coats of primer,
• old adhesive residue that has to be ground off or removed,
• a cold, damp room where everything dries more slowly,
• work in an occupied home, where belongings have to be covered and moved.

Coordinating with other trades

Bricklayers, plasterers and plumbers working in the floor must be finished before this job. Afterwards, the floor layer with levelling compound, the tiler or the floor coating specialist usually follows. In practice it often happens that the electrician chases a channel near the floor after priming and covers the corners in dust again. Agree the order so that nobody makes dust in the room after priming. Nobody should walk on primed areas in dirty boots or stack bags of material on them.

• Priming over dust. The primer binds only the top layer of dust, and a weak layer stays underneath. Later the compound peels off together with the primer or sounds hollow when tapped.
• Puddles of primer. A glossy film forms and the compound bonds poorly to it. It shows as peeling where the primer pooled.
• Wrong type of primer. For example, a standard primer on old tiles. The compound separates from the smooth surface, often only once the floor is in use.
• Silicone instead of acrylic. Neither compound nor coating sticks to it, an open joint forms in the corner and the coating runs off it.
• Sealed movement joint. If a rigid material fills a spot where the floor needs to move, the screed or compound cracks along the walls.
• Sealant in a deep gap without backer rod. The acrylic dries slowly, shrinks and leaves a dip or a crack in the corner.
• Open pipe penetrations. Compound runs into the ceiling below or into the neighbour's flat, and a sunken patch is left around the pipe.
• Not waiting long enough. Compound poured onto wet primer forms bubbles and tiny holes, known as pinholes.

A handy homeowner can manage this job with patience and by following the technical data sheets of the materials. The key is cleanliness and choosing the right primer for whatever goes on the floor next. With solvent-based or epoxy primers, ventilate well and wear gloves and eye protection.

1. Find out what will go on the floor next and buy a primer from the same system.
2. Remove the skirting, adhesive residue and loose pieces of screed.
3. Tap and scratch the subfloor and scrape out any weak spots.
4. Test absorbency with a drop of water and choose the primer dilution accordingly.
5. Sweep the floor and vacuum it thoroughly, including corners and gaps.
6. Stir the primer and roll it on evenly over the whole area, without puddles.
7. Let it dry as the manufacturer specifies and keep out of the room in the meantime.
8. Fill larger holes at the walls and pipes with mortar, and push backer rod into deep gaps.
9. Gun the acrylic into the corners and around penetrations and smooth it with a damp spatula.
10. Wipe off any excess straight away with a damp cloth.
11. Once the sealant has set, apply a second coat of primer if the system requires one.
12. Before the next layer, check that the surface is dry, clean and free of glossy patches.

If you are unsure what kind of subfloor you have, especially with anhydrite screed or old tiles, it makes sense to ask a floor layer to assess it. A priming mistake cannot be fixed later without removing the compound on top.

Check the work before the next trade arrives. After that you will not see anything. Walk the room in good light and focus on these points:

• The surface is evenly primed, with no missed patches and no glossy puddles
• No dust comes off on your palm and the surface does not crumble
• The corners are sealed continuously round the whole perimeter, with no breaks or holes
• The sealant is smooth, sticks to both floor and wall and does not peel off easily
• Pipe penetrations are closed and there are no openings into the ceiling below
• Movement joints and the space for the perimeter edge strip have been left free
• There is no sealant or primer on walls, door frames or pipes
• The tradesperson has told you which materials were used and when the next trade can start