After removing the thru hull, check the condition of the inside edge of the hole to make sure it is not mush.
If it is you need to get the rot out and re-epoxy the coring around the hole. For resealling the cleaned out voids, use epoxy thickened with a filler made to the consistency of thick peanut butter If it is decent, you should apply a coat of thin epoxy to seal up the inside edge. You want the finished, sealed hole to be a tad wider than the thru hull fitting. Let the epoxy cure
If you cut and fit masking tape around the edge of the fitting, removing the squeeze out will be a lot easier. To make the masking tape pattern, apply the tape around the hole without the fitting installed and make sure the tape covers an area wider than the flange. Insert the fitting temporarily and draw a line around the fitting's edge onto the masking tape.Using a razor, cut along the line and peel off the tape that would be under the fitting's flange. Now you will have masking tape around the fitting to catch the squeeze out.
To install the thru hull, apply a liberal coating of 4200 to the flange and the first inch or so of the fitting under the flange. Normally you would use 5200 but since you are having issues and may need to remove it later, use the 4200.
Here is the important part for installation: Apply a bit of 4200 on the flange of the nut and screw the nut on snuggly but not super tight. You want the 4200 to squeeze out on both sides but you don't want to squeeze out all the adhesive.
Let the fitting sit for a day or so and then retighten a bit tighter. By now the 4200 has set up and partially cured and will not completely squeeze out.
Good luck