For a large gap between your skirting (baseboard) and the wall, first bulk out the void using backing material like foam backer rod or tucked kitchen towel, leaving space at the top. Next, apply a flexible, paintable gap filler or expanding foam, smooth it flush, and paint once fully dry.
Expanding foam would work and might help bond the skirting to the wall, but could get messy. Another possibility is silicone or a no more nails type of adhesive, with a gap like that you can get the nozzle of a cartride in to finish below the top lip of the skirting.
An easy fix would be to buy wooden strips that are 2.5-3 cm thick and secure them in place with adhesive and a nail gun, making the surface even. Sand down your wood strips once they are in place to ensure they are flush with the wall. Next, test the surface by putting your architrave against them.
Acrylic compound – is perfect for filling gaps in skirting boards, as it is a flexible material that can be easily applied, as well as matching the color of the skirting boards.
After having our floors sanded and varnished, we didn't want to risk messing them up again with caulk, so we decided to use some window sealing tape to fill the gaps. The tape we used is called weatherstrip and seals gaps from 2-8 mm.
Joint compound is easy to apply, dries quickly, and is inexpensive. It is available in both pre-mixed and powder form, and you can choose between different drying times, depending on your needs. Putty is another gap filler that is popular among professional painters.
Wood filler does not shrink after drying like caulk. And you can shape the wood filler after it dries (over fill the joint, let it dry, then sand the angle to 90 degrees). This may be necessary here because that joint is so loose. You will see divots after dried if you use caulk .
Silicone is not universally "better" than caulk, but it is vastly superior for wet environments like bathtubs and sinks. The best choice depends entirely on your project's environment and whether you need to paint over the sealed area.
Wood filler should generally be used for gaps no wider than 18one-eighth18 inch. For anything wider, the filler is highly prone to shrinking, cracking, and falling out. Instead, you should use wood slivers ("dutchmen"), pocket hole plugs, or a specialized wood epoxy to fill the space.
Whether your skirting is flush or slightly gapped, the most important thing is a smooth, sealed edge between the wall and floor.
Filling large gaps (wider than 14one-fourth14 inch) with caulk requires a backer rod (a squishy foam tubing). Simply filling large spaces with caulk causes it to sag, shrink, and crack as it cures.
A self-adhesive mesh patch works well up to six inches if there's solid overlap and careful compound use. Edges should be feathered a couple inches past the patch. Use multiple coats and sand at 150 grit for a smooth finish. For holes bigger than six inches, a different approach is needed.
Use Caulk to Fill the Gap
You'll want to use a paintable latex silicone caulk to fill the gap.
Professional painters use lightweight spackle for drywall and solvent-based or acrylic wood fillers for painted trim. They select these materials to prevent shrinking, cracking, and to achieve a flush finish before painting.
It is crucial to avoid caulking weep holes, window trims, and movable parts. Before applying the new caulk, you should carefully remove the old caulk using the appropriate tools and techniques. Choosing the right caulk for specific applications, like interior vs. exterior use, is essential.
For large gaps, traditional water or solvent-based wood fillers are not recommended as they shrink and crack over time. Instead, choose two-part wood epoxies or epoxy putties. These chemically cure to bond permanently, will not shrink, and can be sanded, drilled, and shaped just like real wood.
As a general rule, any gap wider than 14one-fourth14 inch (and deeper than 12one-half12 inch) is too big to fill with caulk alone. Attempting to caulk a massive gap will usually result in the caulk sagging, shrinking, cracking, or falling through the opening as it dries.
Understanding Wood Putty vs. Wood Filler. You can easily mold wood putty, and it's the best product for filling cosmetic holes. On the other hand, wood filler dries stronger and is a better option for structural repairs.
Most silicone isn't paintable, so you wouldn't want to use it in areas that would get painted at any point. If you're looking at exterior products, polyurethane caulk is a good option as well. Most caulks will do that if not painted.
Silicone caulk, is often referred to as rubberized silicone caulk, remains flexible for most of its lifetime without peeling, cracking, or distorting. It will maintain and keep a waterproof barrier against moisture much longer than acrylic caulking as a result and typically is more expensive.
You can caulk over existing caulk, but it is not recommended. New caulk adheres poorly to old caulk, which can cause the new seal to peel and lock in hidden mold. For a lasting fix, always remove the old caulk completely.
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DAP Plastic Wood Professional Wood Filler: A fast-drying, strong filler that can be molded and sculpted. It's stainable and paintable and comes in various wood shades. 3M Bondo Wood Filler: While known for car body repair, Bondo also makes a strong, solvent-based wood filler suitable for larger repairs.
For gaps wider than 14one-fourth14 inch in wood trim, standard caulk or wood filler will shrink and crack. Instead, use a two-part wood filler (like Bondo) for painted trim, or glue in custom wood shims/scraps for stained trim, followed by careful sanding and finishing.