If you want to improve the overall appearance of a paint job it's a good idea to caulk between the walls and trim moldings to fill any gaps, before applying the finish paint. To help the caulk adhere to new, bare wood and drywall, prime these surfaces first, and when the primer is dry, use an acrylic latex painter's caulk to fill the gaps.
Take care when choosing your caulk to use a paint-able acrylic latex or silicone enhanced acrylic. Don't use pure silicone for painting, it tends to repel both latex and oil-based coatings causing them to separate while wet and to peel when dry. Pure silicone should only be used for non-painted surfaces like tubs, showers and tile or for weather-proofing to block drafts.
The first step to re-caulking is to remove any old, loose material from the gap between the two surfaces. In some cases a putty knife can be slipped under the old caulk and the whole bead can be removed at once. Otherwise, wedge a putty knife into the crack and drag it down the length a few times to loosen the wall material and old paint so it can be removed.
If the old caulk is cracked but not loose, trim it out with a sharp drywall knife. Cut cleanly along both sides of the crack, but avoid cutting into the wood trim or the wall surface. The gap to be filled should be no wider than about 1|8in when using caulk alone. If you have a wider gap than this, fill it with something else and then caulk it. You can use a strip of wood to fill extra-wide spaces or fill the gap with spray-foam insulation first and then re-caulk it.
Cut the applicator tip of the caulk tube at an angle and far enough down to make about a ¼in. opening. There's a foil seal where the applicator tip meets the tube, pierce this with a long nail or the piercing rod included on some caulk guns. Insert the tube into the gun and push the plunger down with your hand until it makes contact with the push plate inside.
Engage the trigger until caulk begins to flow from the tip, watch for excess that may run quickly from the tube. If this happens, turn the pen and pull back on the plunger to release the pressure. If it keeps running even with the pressure off, hold a rag over the tip and then begin filling the crack as quickly as possible until the flow of caulk slows and stops.
Point the applicator tip into the crack starting at one end. Pull the gun along to leave a bead of caulk that just fills the space between the two surfaces. Do a few feet and release the pressure on the tube by pulling back on the plunger.
Some pros recommend pushing the gun along the crack rather than pulling it as illustrated here. We don't recommend the pushing method because it can be difficult to monitor the flow of caulk, causing gaps to form in the bead. Going back to fill the missed spots can cause problems with excess caulk and smears on the surrounding surfaces.
Wet an old, cotton t-shirt or similar cloth and squeeze out the excess water. With your index finger inside the cloth, run over the bead to smooth it out while pushing caulk deeper into the crack. Turn the cloth to a clean spot frequently to avoid a buildup and rinse it as needed to remove collected caulk and avoid spreading it to unwanted areas.
After you've smoothed the bead go back and wipe any excess from the wall and trim surfaces before it can dry. When you've finished be careful not to disturb the until it has time to set and cure. Acrylic latex caulk will set (become firm) in about an hour and cure (become hard) in about 24 hours. For the best finish, wait for the caulk to cure before cutting-in around the corners with a paint brush.
The cracks that commonly appear around a shower or tub are caused by the weight of water and bodies forcing the floor to sink with every use. When the tub is empty again it returns to its original level, leaving cracks around the base. To fix this problem and compensate for future movement use a flexible, pure silicone caulk to fill the seams.
First, remove any cracking or loose caulk. If it's very loose and coming away at the edges it may be possible to remove most of it by running a plastic putty knife under the bead as you pull it free. To remove more stubborn caulk, begin by working a plastic or wooden scraper, back and forth, along the seam to remove as much as will come free and then use a solvent to remove the rest. You can scrape ceramic tile and porcelain with a metal putty knife or razor-blade scraper, but fiberglass tubs and showers should be treated with plastic or wood to avoid damage to the finish.
When all the loose caulk is removed, soften what remains using denatured alcohol. Soak a soft, absorbent rag with plenty of alcohol and apply it to the seam and surrounding edges. Let the solvent sit for several minutes and then use the rag to rub off as much caulk as will come free. Soak the rag again, wrap it around your plastic or wooden knife and use it to force the rest off. This can take a while and repeated soaking and scraping to get it all.
On ceramics or porcelain, you can use a razor-blade glass scraper and steel wool to finish cleaning the surface. If you're dealing with fiberglass or metal parts, use a plastic kitchen scrubber to remove the last bit of residue, don't use steel wool or other hard abrasive. To finish, wipe the seam with a fresh rag soaked with alcohol and let it dry for about 10 minutes before re-caulking.
You can buy silicone caulk for tub and tile in a tube that resembles toothpaste, or you can get it in a standard tube that will fit a caulk gun. Cut the applicator tip on the tube at 45° and pierce the seal at the base using a long nail. Soak an old, cotton t-shirt or similar cloth in water and wring out the excess. Use the rag to smooth out the bead and wipe off excess caulk. Water won't dissolve the silicone, but it will help to smooth the surface.
Begin working at one corner and fill the gap moving all the way to the next corner without stopping. Go slowly enough to fill the gap, but not so slow that it becomes overfilled with caulk to avoid excess that can make it hard to get a clean, smooth finish. Don't stand in the tub or shower while working to avoid overfilling the gap, causing caulk to squeeze back out when you move and ruin your new finish.
Use the wet rag to smooth out the bead while at the same time removing the excess caulk. Turn the rag frequently to avoid spreading the collected caulk to unwanted areas. When you have wiped over the whole bead with the rag, wet your index finger with water and go over it lightly to get a nice, smooth finish along the seam. Wipe the collected caulk from your finger onto the rag to prevent spreading it around. Allow the caulk to set for an hour or two before touching it and don't use the shower for 12 to 24 hours to give it time to cure completely.