If it's cracked, peeling, or otherwise in bad shape, it's best to remove it entirely before applying new caulk. This will help ensure a tight, water-resistant seal. If the old caulk is in good condition, you can apply new caulk right over it.
Absolutely remove the old caulk. The old stuff is obviously breaking apart, and will continue to do so. If you put new caulk on top, it's going to fail as the old caulk continues to break apart and no longer adhere to the surface below.
The overarching reason why new caulk should never be installed over old caulk is because it simply does not produce long-lasting results. The reasons why depend on what has happened to the previous caulk line. In the case of caulk that has gotten moldy, recaulking over this material will not stop the mold problem.
Yes, new silicone sticks to old silicone if it's mostly clean like in your situation. It's not as good as doing it right the first time, but it'll still be water tight. If it's old silicone with a layer of soap scum and mold like in a shower it's best to cut out as much as possible and start over.
If it's cracked, peeling, or otherwise in bad shape, it's best to remove it entirely before applying new caulk. This will help ensure a tight, water-resistant seal. If the old caulk is in good condition, you can apply new caulk right over it.
Never apply new silicone sealant over old sealant as, in most cases, the old sealant would have fallen away or split meaning that, no matter how much new sealant you apply, the leak will persist. Not to mention, applying a new sealant over old will look incredibly messy and unappealing.
However, you can effectively caulk over caulk as long as the bottom layer of caulk isn't damaged or moldy. The old caulk should also be clean, dry, and free of oil for the best adhesion.
The Grout Medic not only recommends but exclusively uses 100% silicone caulk for all our recaulking projects. Silicone caulk is more flexible, repels water, and lasts longer than other mixtures.
You can't rely solely on caulk when dealing with gaps more than a ¼ inch wide and deep. It would help to fill the gap with something else before caulking over it.
You can recaulk over old caulk, but you should never do it.
Our recaulking experts will remove every bit of your icky, failing caulk. Then, they will add an anti-mold treatment to eliminate mold and mildew and fight future mold and mildew growth. They will apply a 100% silicone caulk, which resists shrinking over time.
Cracking - caulk is unfortunately prone to cracking, which sometimes occurs when the caulk is not flexible enough for the surface it is filling. Crazing - when paint is applied over caulk, it can sometimes lead to the paint to peeling & flaking.
Sanded caulk has particles in it that help it adhere to wet surfaces and expand in larger gaps without cracking. It has a grainy texture and appearance. Use sanded caulk any time you use sanded grout and in joints 1/8-inch or wider.
POSSIBLE CAUSE
As the caulk dries, it shrinks and causes the paint film to stretch and crack. Applying caulk when the air, surface, or caulk temperature is below 40 degrees F, and the caulk is not designed for these conditions. Caulking joints in excess of 1/2" in width or depth.
Generally, the old caulk must be removed before applying new caulk to a residence, condo or commercial building (in some cases, it is impossible to remove the old caulk).
For best results as you figure out how to use caulking tools: Lightly drag the caulk finishing tool over the bead to smooth it. Alternatively, you can use your finger. Moisten your fingertip in warm soapy water and drag it with light pressure along the length of the bead.
To smooth silicone caulk, just lay down a nice bead with your caulk gun, then spray a mist of rubbing alcohol on the bead. (The spray head from many different products will fit on the alcohol bottle.) Now, prepare yourself for magical results as you smooth the bead with your finger.
Thicker applications of caulk will take longer to cure. It's important to apply caulk in thin, even lines unless the job specifically requires a thicker bead.
Generally, silicone is most effective for small to medium-sized gaps, typically ranging from ⅛ inches to ½ inches. Attempting to seal large gaps with a single application may result in collapse or shrinkage. When facing wider gaps, epoxy-based sealants or specialized gap-filling products may be a better choice.
Can I apply new silicone atop, or alongside old silicone caulk? It is recommended that any old silicone is removed. While new, freshly applied silicone can be applied & will bond to old silicone – the bond isn't as strong as if it were applied to a clean surface.
Silicone is a rubber-like elastomer with high-temperature resistance. It is difficult to bond, and the only adhesives which adhere well are silicone based adhesives and cyanoacrylate adhesive after the silicone has been primed with Permabond POP.
While a single layer of silicone sealant may suffice in many cases, applying two layers can offer several advantages. Firstly, it can enhance the durability and longevity of the sealant, making it more resistant to wear and tear.
Can I add additional uncured silicone rubber to a silicone mould that is already cured? As long as the surface of the cured rubber is kept dust and grease free, more silicone rubber of the same type can be poured on top and will bond with the surface. There will be no detectable join.