The best gutter sealer for end caps and miters is Rusco 12-3 self leveling sealer. This nitrile rubber sealant sealant remains soft and flexible and does not become brittle like the some sealants do. (Contains No Silicone) allowing excellent contraction and expansion with temperature change.
There are so many gutter sealants on the market proclaiming to seal Gutters but the only one that I use and can personally recommend is a butyl rubber sealant manufactured by the Ruscoe Company in Akron Ohio. For rain-gutters long term water holding reliability end caps and corners are only as good as the sealant used.
Ruscoe 12-1 seam sealer and Rusco 12-3 self leveling sealant are the two consistencies of nitrile rubber sealants that I use for my gutter installations. This sealant remains soft and flexible and does not become brittle over time like the some sealants do. (Contains No Silicone) allowing excellent contraction and expansion due to temperature change.
Butyl rubber sealants are an excellent choice for sealing or repairing gutters. The apply well in cold or hot temperatures, adhere quickly and remains flexible as it cures. These features mean the sealant will move with the gutter as it expands and contracts due to temperature changes and maintain a watertight seal.
Gutter sealants that contain silicone will eventually fail because the solvents in them will evaporate and the silicone will shrink and pull away from the gutter joint. The silicone sealants also lack adhesion strength, so when the temperature changes from hot to cold, there will be movement in the Gutters and the silicone based sealants do not have a very good resistance to this movement.
Ruscoe 12-1 seam sealer is designed to seal openings of less than 1/8 inch and mainly used to seal corners due to its thinner consistency. Ruscoe 12-3 self leveling sealer is designed for sealing opening greater than 1/8 inch and mainly used to seal endcaps due to its thicker consistency. Both of these gutter sealants are effective sealants against water.
All three seams in the gutter corners (back, bottom and front) must be completely sealed.
Here's how I seal the corners. Start at the top of the back corner and with a back and forth movement of the sealant gun of about one inch on both sides of the seam, work your way down to the bottom and back up the front side of the gutter. So you should end up with about an inch of sealant on each side of the seam. Make sure that all the rivets or screws are completely covered with sealant.
The gutter end caps must also be completed sealed. After the end cap is crimped onto the gutter, inject sealant into all three seams, then wet your finger and press the sealant into the seams for a water tight seal.
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