Jim-
If mildew staining the existing sealant beed is your main issue (vs. cracking, shrinking and leaking) try scrubbing with a Mister Clean scuff eraser sponge.

Paul

I need to do the same, but...

My comapnion hatch hood is off the boat for the winter and I am replacing most of the core. When shopping for boats many had water in the core of the hatch hood. If the caulk even 'slightly' looks like the edge might be letting go it can lead to problems. The screws are another area that is not the best design, so those will get solid glass. The lazarette piano hinge is another area given little thought....20 screws right into the core....hmmm

On the plus side, all the deck area around the chainplates on my '05 (newer design ball and socket) is solid glass - a good suprise. The surveyor said they were solid when sounding the hull, but I didn't believe him ;-)

edit: was thinking sikaflex 295-uv caulk but was going to research more and call catalina as well....

I like butyl - the best is from compass marine. Marinehowto.com. The posts there are great at showing how to use it. But spoiler - champher the holes in the deck to provide more volume for the sealant. In really severe areas, I do the routing, epoxy fill and redrill ( helm guard, stantions...)

I'll be following this discussion.  I have a phantom leak somewhere, and I suspect it is the main hatch cover.  During the off season, I get water dripping from the port dorade into the dry storage locker.  I have rebedded the dorade twice and it still leaks.  But during the sailing season, it doesn't really leak at all.  The only difference is that I have a dodger installed all summer, which covers the main hatch.  

dlincoln3: I'm experiencing the same issue. I recently removed the teak rails and hatch cover. I found redundant screw holes adjacent to most of the ones actually used. it appears the rails and hatch cover may have been slightly out of place?? when cover and rails were re positioned the redundant holes were not sealed and some were right at the edge of the flange so that a vertical open crack exists (happens to be in line with cowl vents boh sides) that allows any water that gets in the sliding hatch groove to leak down to cowl vent opening. only leaked sometimes after heavy rain or lengthy beat with heavy water over bow to companionway. next spring I'll seal all holes with epoxy, redrill only needed ones and seal with butyl.

MkI and MKII are different in the hatch cover area - the hatch civer for the MKII needs a sealant between the hatch hood and the surrounding deck area (the hatch hood sits flush with the deck). Love butyl tape, but for this application it's a no go.

I did think of the hand hold. The PO rebedded that, and I'm assuming (maybe foolishly) that it's still OK. I rebedded the rope clutch next to it. The only other possibilities are that hand rail, the cabin top winch, or the hatch hood. I guess I have to make the time to rebed them all and see if that solves it.

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As soon as I'm done with my core replacement every single hole in the topside of the boat is getting drilled and filled. Rotten core from leaky junk is an absolute pain in the butt to fix. On the flip side, the boat is getting a fresh coat of paint and some kiwi grip non-skid. 

If in doubt, rebed all the things!