Short answer: If silicone does not hold on an acrylic bathtub, it is almost always down to one of three causes. First: the wax or release-agent layer that sits on sanitary acrylic from the factory and prevents any adhesion – it must be removed completely before jointing. Second: too much movement, because acrylic bathtubs deform considerably more than steel or cast-iron tubs when they are filled and when the temperature changes. Third: a joint that is too narrow and bonded to the joint back as well. The most important trick that almost nobody knows: fill the bathtub with water before jointing and only empty it again after curing.
Why acrylic is different from steel or ceramic
Sanitary acrylic has changed bathroom planning – the material can be formed into almost any shape under heat, is warm to the touch, light and comparatively inexpensive. For the silicone joint, however, it brings three properties that tubs made of enamelled steel or cast iron do not have.
It expands greatly. The coefficient of thermal expansion of acrylic is many times that of ceramic or steel. When hot water runs in, the bathtub grows measurably – and then shrinks again.
It gives way under load. A filled bathtub with a person in it sinks visibly, especially if the bath support is not supported over its full area. This movement ends up directly in the connection joint.
It is treated at the factory. The smooth, easy-care surface owes itself to a thin wax or release-agent layer from production. It is invisible, cannot be wiped off – and silicone does not adhere to it.
Cause 1: the wax layer
This is by far the most common reason for a joint that comes away on the bathtub side after a few weeks. The joint looks completely intact; it just no longer holds. Technically this is called adhesion failure: the sealant has come away cleanly from one flank without tearing itself.
You can recognise the case by the bathtub flank being bare after the joint is pulled off – no silicone residue, no adhesion. On the tile side, by contrast, material generally stays stuck.
How to remove the layer
Rub the bonding surface on the bathtub rim with isopropanol or a petroleum spirit and a lint-free cloth – do not just wipe, but work with light pressure and change the cloth several times. Washing-up liquid expressly is not enough: it removes grease, but not a wax layer, and itself leaves a surfactant film. Then leave to flash off completely. On heavily treated surfaces, a clean primer helps additionally, which cleans and improves adhesion.
Cause 2: the bathtub moves
Even a perfectly prepared joint tears if the movement is greater than it can absorb. And that is exactly the rule with acrylic bathtubs if corners were cut during installation.
The bath support decides. An acrylic bathtub that stands only on feet works considerably under load. A full-surface bath support made of rigid foam, which supports the bathtub over its entire area, reduces this movement drastically. For tubs made of sanitary acrylic it is not optional but the actual construction standard.
The wall fixing must be load-bearing. If a bathtub is fixed to a light pre-wall or stud wall whose cladding gives way, every movement of the user is transmitted to the joint. A solid substructure with cross braces belongs here – or you do without the acrylic bathtub.
Decouple structure-borne noise. Before installation, a closed-cell foam tape or a PE backer rod is usually stuck onto the bathtub flange so that the bathtub does not touch the wall directly. That not only dampens noise but also defines a clean joint space.
Cause 3: the joint is built wrongly
Let us work it out. A high-quality sanitary silicone absorbs around 25 percent of its joint width in movement. With a 10 mm wide joint that is 2.5 mm. With a 5 mm wide joint, only 1.25 mm.
A filled acrylic bathtub with a person in it sinks lightly by several millimetres, depending on installation. With a narrow joint the sum simply does not work out – the joint has to tear, whatever product is in it.
On top of that comes adhesion to the joint back: if the sealant adheres not only to both flanks but additionally to the joint back, it cannot expand freely. This is prevented by a closed-cell PE backer rod or a bond-breaker tape. The relationships are described in detail in our guide How wide may a silicone joint be.
The trick that makes the difference
Fill the bathtub before you joint. Fill the bathtub with water to just below the overflow and joint in this state. The water presses the bathtub into its lowest position – so the joint is built in the most unfavourable state, at maximum joint width. Only drain the water once the silicone has cured through completely, that is after at least 24 hours. After that, the joint is only compressed when filling instead of pulled apart – and silicone tolerates compression much better than tension.
The same idea applies to shower trays: load them if possible, but at least step into the tray before jointing and pull the joint in this position.
Step by step: jointing an acrylic bathtub
1. Remove the old joint completely
Work with a plastic scraper or a blunt joint knife, no utility knife – acrylic scratches immediately, and in a waterproofed bathroom the blade cuts through the waterproofing beneath the tiles. Swell residues with a silicone remover and push them off. New silicone does not adhere to old silicone.
2. Prepare both flanks
Degrease the bathtub rim with isopropanol and rub off the wax layer, degrease the tile side as well. Leave both to dry completely. With natural stone instead of tiles, a natural stone primer and a natural stone silicone belong in that place.
3. Backfill
Press in closed-cell PE backer rod, with a diameter around 25 percent larger than the joint width. Do not puncture it. If the joint is too shallow for a rod, a PE bond-breaker tape goes onto the joint back.
4. Fill the bathtub
To just below the overflow. With shower trays, load the tray or step into it.
5. Joint and smooth
Cut the nozzle to the joint width, work in sections of one to two metres and smooth immediately, before a skin forms. Spray smoothing agent sparingly onto the squeegee, not into the joint. If masking tape is used, pull it off immediately after smoothing.
6. Leave to cure, then empty
At least 24 hours, longer in cool or dry air. Only then drain the water. During this time do not use the bathtub and ventilate the room.
Which silicone for acrylic bathtubs?
| Requirement | Why |
|---|---|
| Neutral-curing | acetic acid can attack acrylic and adjoining metals |
| Sanitary silicone, fungicidal | permanent damp, mould exposure |
| High permissible total movement | acrylic moves more than steel or ceramic |
| Approved for sanitary acrylic | stated in the technical data sheet, not on the cartridge |
| Additionally for natural stone | natural stone silicone without migrating plasticisers |
A note on the confusion of terms: the question “sealing a bathtub with silicone or acrylic?” means by “acrylic” the sealant acrylic, not the bathtub material. The answer is clear: silicone. Acrylic as a sealant has hardly any movement capability, is not permanently water-resistant and is unsuitable for wet areas. It belongs on connection joints that are painted over – not on the bathtub.
The installation decides the joint
It sounds laborious, but it is the heart of the matter: the most durable silicone joint on an acrylic bathtub is not made with the cartridge, but when the bathtub is installed. Four points determine how much movement the joint will have to withstand later.
Full-surface bath support
A support made of rigid foam that reproduces the bathtub contour completely takes the load over the entire area. A bathtub that rests only on feet and at the rim bulges in the middle under water and weight – and pulls the joint apart at the edges.
Load-bearing wall construction
With pre-wall installations and lightweight walls, cross braces and double cladding belong behind the bathtub. If the wall gives way, the best bath support is no help. Where a solid construction cannot be achieved, a bathtub made of enamelled steel is the more robust choice.
Structure-borne noise decoupling at the flange
Before inserting, a closed-cell foam tape or a PE backer rod is stuck onto the bathtub flange. This prevents direct contact with the wall, dampens noise and incidentally creates a defined joint space instead of a random gap.
Bath edge sealing tape under the tiles
The actual moisture protection sits not in the visible silicone joint, but beneath it: a bath edge sealing tape is laid into the waterproofing before tiling and bridges the connection. If the silicone joint tears later – and at some point it will – the tape catches the water. Without this tape the silicone joint is the only barrier, and that is a poor construction.
Shower trays: the same rules, tighter tolerances
With acrylic shower trays, the added complication is that the user stands directly on the surface. The point load is higher than in a bathtub, where the weight is distributed through the water.
That is why the following applies particularly here: full-surface support or mortar-bedded installation, not a pure feet-only construction. For jointing, step into the tray and pull the joint in this position – with a level-access shower surface, load it accordingly.
A second point concerns cleaning: on shower trays the joint is in constant water run-off and is more heavily exposed to shower gel and lime soap than at a bathtub rim. Regular squeegeeing and wiping dry extends its life considerably – how to do this with reasonable effort is explained in our guide Cleaning silicone joints.
The joint is a maintenance joint
A point that saves a lot of frustration: a sanitary joint is not a permanent component. It is a maintenance joint with a limited service life – depending on use, ventilation and cleaning, five to ten years is normal, and less on a heavily used shower tray.
That it eventually has to be renewed is not a defect and not a sign of poor material. It only becomes a defect if it fails after weeks or a few months – then something is wrong with substrate preparation, joint geometry or installation.
That is exactly why the bath edge sealing tape under the tiles is so important: it makes the planned ageing of the silicone joint harmless. Without it, a normal maintenance case becomes water damage.
The most common mistakes
Removes grease, but not the wax layer – and itself leaves a film. Isopropanol or petroleum spirit are mandatory here.
The joint is built in the unloaded state and pulled apart at the first full bath. The crack then appears within the first days.
Scratches in the acrylic stay visible and become dirt traps. In waterproofed bathrooms the waterproofing under the tiles is also damaged.
Fresh silicone does not adhere to cured silicone. The new joint looks good for a few weeks and then comes away together with the old one.
If a joint tears three times in a row, it is not down to the silicone. Then the bathtub is moving too much – and that is solved at the bath support, not with the cartridge.
Frequently asked questions
Why does silicone not hold on my acrylic bathtub?
Mostly because of the factory wax layer on the sanitary acrylic, which does not come off with washing-up liquid. The second most common cause is too much movement due to an inadequate bath support, the third a joint that is too narrow without backfilling.
Which silicone for acrylic bathtubs?
A neutral-curing, fungicidally finished sanitary silicone with high permissible total movement that is expressly approved for sanitary acrylic in the technical data sheet. Acetic-curing products do not belong on acrylic.
Must the bathtub be full when jointing?
Yes, that is the most effective single step. The filled bathtub sits in its lowest position, so the joint is built at maximum width. After curing, in use it is only compressed instead of stretched.
How wide must the joint at the bathtub be?
At least 5 mm, better 8 to 10 mm. With an acrylic bathtub with noticeable movement, rather at the upper end – a narrow joint cannot, by calculation, absorb the movement that occurs.
Sealing a bathtub with silicone or acrylic?
With silicone. The sealant acrylic has hardly any movement capability and is not permanently water-resistant; it belongs on paintable connection joints in dry areas, not on a bathtub.
What should I do if the silicone comes away from the shower tray?
Remove the old joint completely, degrease the tray flank with isopropanol and rub off the wax layer, backfill, load the tray and re-joint. If the joint comes away again anyway, it is down to the installation – then the bath support or the wall fixing has to be improved.
How long must the silicone cure before I can shower?
At least 24 hours, in cool or dry room air rather 48. Silicone cures from the outside in with the humidity of the air, about 2 mm per day – with an 8 mm deep joint the surface is long since firm while the core is still working. Anyone who loads it too early displaces the still-soft compound.
Can I renew the joint myself or does it belong in professional hands?
Renewing a connection joint is a classic DIY job and, with some care, easy to manage. It becomes a professional matter where the cause lies in the installation: a sagging bath support or a yielding pre-wall cannot be fixed with a cartridge.
Why does the joint on my acrylic bathtub keep turning black?
Mould feeds on soap and care-product residues on the joint. If it returns quickly despite cleaning, the mycelium already sits in the material – then only renewal helps, with a fungicidally finished sanitary silicone and better ventilation.
Does a primer help on acrylic?
A clean primer cleans and noticeably improves adhesion on plastics and coated surfaces. It does not replace degreasing but comes after it – and it is the sensible additional measure if a joint has already failed once at the same spot.
