When you get into your bathtub and notice a squishy or spongy feeling beneath the tub liner, you know you've got water leaking between the two surfaces.
This is bad news. Untreated, water under a tub liner will lead to further leaking, mildew and mold, and eventually the dissolution of adhesive and paint. Moreover, standing water will eventually become stagnant and create a strong, icky smell.
Step 1 - Check the Warranty
Before attempting to fix the problem yourself and risking causing a "tampering" issue with the supplier later, call or contact them and inquire about the possibility of a replacement.
Check on the liner's warranty—they often come with a lifetime warranty that can even be transferrable by some manufacturers. So even if the liner was acquired by the previous owners, it's a good idea to get in touch with them and find out more about the liner's origins.
If the warranty isn't an option, the next step is to decide whether to replace the bathtub with a new one or attempt to repair the existing one.
Step 2 - Find the Source of the Leak
Before taking action, it's a good idea to find out everything about the acrylic liner. This is simply a layer of acrylic molded to the old bathtub and glued or taped on. It generally covers all the tub's surfaces, inside and outside, and is caulked along all of its seams.
If you have water infiltrated between the two layers, give your bathtub a good thorough inspection to find out where the leak is happening.
2.1 - You can start by checking all the seams around the liner and the vertical wall surround. Look for cracks or holes in the caulking that would allow the water to penetrate and build up.
2.2 - Check all over the liner surface for cracks. Shells don't necessarily match the contour of the original bathtub perfectly. This can cause the acrylic to flex and, over time, form cracks where water can get through.
2.3 - If the caulking is in good condition and the liner's intact, there is a possibility that the leak originates at the drain or the overflow joints. By removing the overflow cover you'll be able to verify the seal between the acrylic and the tub wall.
Make sure a generous amount of silicone adhesive is providing a good solid bond between the two surfaces. The drain can be taken apart by first removing the stopper. With a Castle wrench, you can then remove the drain insert that threads through the liner, the tub, and the gasket before screwing into the drain shoe.
If you pull carefully on the liner, you'll be able to see if an appropriate layer of silicone adhesive has been used and properly spread. Since, for obvious reasons, this area of the liner is at its thinnest, proceed carefully to prevent any tearing damage to the liner.
Step 3 - Fixing the Leak
The most expensive solution if you decide to get the faulty bathtub repaired is obviously getting a new liner installed, which would include the removal of the existing liner which has likely been glued to the bathtub.
Glued-in liners have to be cut out or ripped out. Get an accurate estimate if you opt for a new liner to be installed—it might help you decide on a new tub instead.
3.1 - Checking the liner for the possibility of fixing and salvaging the liner will not cost much as far as materials go, except for sweat and effort. Just be aware that your chances of success are no more than 50/50.
Proceeding from Step 2.3 where the overflow cover and the drain have been removed, Get a strong coat hanger with the heavier wire gauge, and bend the end to shape it in an overbent "L-shape" so that the end of the wire makes contact first when pulling straight up.
A similarly bent 1/8 x 3/4-inch (3x19 mm) aluminum flat bar might even serve you better if you have one.
3.2 - Before attempting any further repair, you must first try and remove the stagnant water trapped inside. You must remove as much of the water as you can by standing inside the bathtub and "walking" the water out, or pushing it out while applying pressure down against the liner. Push the water out toward the drain.
3.3 - In this next step, you should proceed carefully as this area is where the liner is at its thinnest and can easily rip or break attempting to pull on it with your makeshift "L-tool."
Inserting the bent portion of the tool under the liner at the drain opening, pull straight up to lift the liner while you keep moving out the remnants of the stagnant water.
By trying to lift the liner, you'll see how far the bonding has left go if at all, and make a final decision on how serious the problem really is. If much of the bond between the liner and the tub has broken down involving the complete removal of the liner, it would probably not be possible without damaging much of the unit.
3.4 - After removing as much water as you can, while a helper keeps the liner raised with the "L-tool," wipe dry everything around the drain opening, and as far as you can between the tub and the liner. You can then apply a generous amount of silicone adhesive between the two surfaces all around the opening.
3.5 - Make sure you apply a good coating of plumber's putty, then reinstall the drain securing it tightly in place.
3.6 - Using the same technique as with the drain (except for the plumber's putty), replace the overflow cover securely in place.