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Once you're ready to go, make sure the area that you're applying the sealant to is CLEAN, DRY and DUST FREE. The caulking material may not adhere properly if there is dust or dirt on surfaces.
APPLYING THE CAULK: Push the plastic tip into the corner or gap to seal. Squeeze the caulking gun trigger and pull towards yourself SLOWLY, gripping and releasing the trigger repeatedly to keep a line of sealant coming out. Slowly make your way down the entire gap until you've covered the entire length.
STOPPING THE FLOW: When you've reached the end of the gap, hold the gun steady and press the release trigger. Pull up the caulking gun off of the bead, being careful not to smear the bead or trail caulking onto other surfaces. You may need to press the release trigger and pull back the compression plate to stop the flow.
FINISHING THE BEAD: You can use a caulking corner finishing tool to smooth out the bead of caulk to a clean finish - there are scrapers and finishing blades available in most building supply stores. You may also use soapy water and your finger to smooth out the bead - the soap prevents your finger from sticking.
With some practice you may be able to go around corners or continue long lines of caulking without stopping. Overlap your stop / start points just a bit to keep the bead size consistent, being careful not to push out too much of the caulking.