The principle behind caulking is to have the surface look natural and for the caulking to become invisible once it is painted.
There are many types of caulking and each product has a specific application. It is important that you choose the proper caulking for your job. The caulking that I will be writing about is for interior cosmetic finishing. (Read labels for information on product) I try to stay away from adhesive caulk and cheap caulking. The caulking you choose should be paintable and should not be painted until dry. I have always found that the better the product the longer it lasts. I have been using the Dap 3.0 recently and have found it out performs anything I have ever used in the past. It is more difficult to work with and if you are a beginner I would stick with the more traditional high quality caulks.
Why should you caulk? Sometimes the trim may be nailed or glued on to an uneven surface resulting in a void. If your trim is painted and not caulked the end job will not look completed and may even look worse because of paint drips that form from the openings. If the trim is to be stained and or varnished no caulking should be used.
Caulking is primarily used to fill in the spaces where two pieces of molding meet and where there is a void between wood trim and drywall. Examples would be window and door casings, crown molding, base molding and chair-rail molding. Spaces will vary in size but most should be less than a ¼ in. If the space is larger you may have to apply multiple applications depending on the viscosity of the caulking. If the spaces are too large a backer rod (circular foam rod) or even a fitted piece of wood can be used to fill in the bulk of space.
Next: two very important procedures should be followed.
1) All surfaces should be primed and dust free prior to the application. A common mistake by contractors and home owners is caulking over spackle or bare wood. Spackle is dusty and porous and bare wood is too porous which will cause the caulking to dry prematurely. Priming is very important because this will prevent poor adhesion.
2) Also be certain that all surfaces have little or no movement at all. Simply press on the surface if it springs back than nail it until there is little or no movement. I run into this problem on just about every job I am on, the contractor may forget to nail some areas or even worse will nail into drywall without hitting a stud. So it is best to push on the surface and feel for movement and correct as needed. Good carpentry will keep the wood tight and will provide a reveal on door and window frames. The casing should be set back at least a ¼ inch from the jamb. If the casing is nailed flush with the jamb, you will have a very difficult time filling the void without it looking sloppy.
You should cut the tip of the cartridge to match the size of the opening and better to be smaller than larger. Cut with a sharp razor at a 45 degree angle. Try to get a clean even cut because if it is a jagged cut the caulking will not flow evenly. Some caulking cartridges may also have an inner seal that may need to be perforated. Next install into caulking gun and squeeze the trigger gently until the caulking begins to come out. Clean the tip and remember to always start with the tip wiped clean.
Start in the corner and work your way across seam in an even long stroke, pulling the trigger gently. You should keep pace with the caulk exiting the cartridge by observing the amount of caulking coming out. Over application can create a messy clean up or if left alone will produce a poor job. Too little caulking with result in voids appearing and will require reapplication. After you have finished each seam take a wet rag and use it to wet your finger and gently press caulk flush with the surface. The goal is to strike the surface leaving it uniform, smooth and with little or no waste. Keep a soapy rag with you to wipe your fingers and hand, do not use a wet rag to smooth the caulking with. Excess caulking should be removed with a small putty knife of your finger. Use of a damp rag may be used, but be careful not to allow water to dilute the caulking. If you remove the bulk of caulking with a rag you will be left with a thin film to bridge the surface and it will crack when it dries. Allow the caulking to completely dry prior to painting. Painting prematurely will produce small cracks in the paint when dry.
Remember the idea is to have the surface look natural so that the caulking is not visible when painted.
Oh before I forget DO NOT USE CAULKING TO FILL NAIL HOLES. Use a wood filler. Caulking will shrink and residue will form on surface and it is very difficult to remove.