Measure the joint before buying sealant
Record the narrowest and widest gap, joint depth, substrate type, movement, water exposure, and whether the sealant must be paintable. Clean and dry the surfaces without damaging a waterproofing layer. Then compare the measurements with the exact sealant technical sheet.
Backer rod is not a universal filler. It must fit without being cut or compressed into a shape that changes joint geometry, and it should not damage a membrane or create a hidden water path. A joint that is moving, damp, contaminated, or structurally open needs diagnosis before cosmetics.
Stop conditions
Pause when the gap is between dissimilar building materials with unknown movement, the substrate is loose or wet, the joint is a structural crack, or the sealant will be exposed to a condition the label does not cover.
Limits
No joint was measured or sealed. This guide does not set a universal backer-rod diameter, sealant depth, or product compatibility.
Sources and related reading
See Also
If you want to move from general advice into actual product choices, start with A Door Rubs at the Top: Hinge, Frame, or Seasonal Movement?, Cabinet Hinge Overlay Measurement: Match the Replacement, and When to Replace Drywall Instead of Patching: Signs, Costs, and Repair Timelines.
For a wider picture after the basics, Pella vs. Andersen Windows: Repair Costs and Maintenance (Head-To-Head) and Klein Tools ET310 Review: A No-Nonsense Circuit Breaker Finder are the next places to read.