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I have always done what CYS has done. Once I pull the old tubes, I let the the drain hole dry out, clean w/pipe brush, then brush/coat the interior of the drain hole with West System - two thin coats.

If you go too thick you will not be able to get the new tube in.

It is impossible for sunlight to hit the interior of the drain holes,especially with the new drain tubes installed.

Install the tubes with the O-rings & 4200, flare the ends & done.