There are many types of caulking from clay ropes to butyl rubber, but for filling cracks under new paint a good
quality latex caulk is easiest and best for most uses.
Never use a Silicone rubber caulk anywhere you plan to paint, even the paintable silicone moves more than
the latex or oil paint and will show the crack in the paint again soon.
Any gap over about a quarter inch needs a foam backer, (or rope, rolled paper, etc.) as filler to back up the
caulking.
If you have a large crack use drywall compound (mud) and drywall tape on indoor drywall, or waterproof spackle
outside to fill it, or replace the cracked board or add a trim board if needed for a better job.
Cut the caulk tube tip at an angle as close to the end as to make a round tooth pick size hole, then use the poker
on the caulk gun or a piece of wire to pierce the foil seal inside the tube at the base of the tip if necessary.
Most people cut this too large, cut it as small as possible with about a forty five degree bevel to the cut.
Caulk guns have a way of releasing the pressure to the tube and or allowing the removal of the tube, some have
knotches on the plunger rod so you just turn the rod to dis engage the plungers ratchet and release the pressure,
and some have a tab around the rod that binds it and the pressure is released by pressing the tab.
Having a paper towel in hand, place the cut end of the tube on the crack so you can pull the tube towards you
comfortably. Press the tip firmly keeping the hole in the tip flat against the crack and slowly squeeze the
handle while moving the tip so the caulking just fills surface of the crack, stop squeezing a little before the end
of the crack to allow the pressure to slow, and release the plunger as you lift the tip. Do not over fill the gap,
you can always make a second pass and I often use two or more passes to fill a larger gap.
Before you place the tip against the wall each time, wipe the tip with the towel and have the towel ready to
catch the drip and wrap the tip when you remove it from the wall.
Any time Caulk starts to build up on the tip, wipe it off.
If you have to you can use a damp cloth, ( paper towels can leave fuzz balls ) to clean latex caulk off the wall
surface or smooth a joint, But a quick light wipe with a clean finger can also smooth a joint.
Don't allow caulk to build up on your finger or the towel. If you keep the tip flat and don't over fill the crack
you shouldn't need to smooth the caulking.
Latex Caulk is good for primed (or painted) clean surfaces, generally under paint. water clean up.
Silicone rubber is flexible and used for water seals in the bathroom or kitchen, between tile and tub or glass like surfaces. Silicone comes in plain original type , good for fish tanks, or mildew resistant which is good for showers, and will kill fish if used in fish tanks and paint able. Use neatly and never allow it where paint will be applied. even the paint able type resists paint some. nothing cleans it, let it dry and cut it loose with a sharp razor.
Butyl Rubber is more flexible and works well on wider joints with more movement needed.
Lexan is a stiffer rubber that cleans better than the others, but is much harder to apply
Poly seam seal, sold in squeeze tubes, used to set sinks, harder finished surface with softer seal. Easier to clean for the kitchen or bath sink, not as good in shower or with standing water. easier to apply than most other caulk Apply the caulking, then smooth with your finger, then a folded smooth wet cloth to clean excess caulk from the sink and counter. Avoid flooding the caulk with water, or gouging into the seam. Then let dry. The water will cause a harder glaze on the sealant.