I would not put the blame for the blisters on the time between bottom paint jobs. I have bottom painted my 1988 C30 three times in the 28-years I have owned her. I pull the boat every 9-years to paint and have had exactly three 1"ø blisters to fix. FWIW, I use Pettit Trinidad paint.
Overbore, epoxy fill, and rebore every hole before reattaching all.... okay so it's better than "remove bolts/screws/nuts, apply sealant and reinstall bolts/screws/nuts" Like this:2026 minus 1995 equals about 31 years. More or less, depending on month finished out. From experience I know that factory sealants (that seal ALL thru-deck fastener penetrations, have a reliable life of about 25 to 30 years at best. Then you can expect water to start invading the core. Best way to counteract this oft-neglected problem is to re-bed ALL the fasteners. i.e. core rot is NOT inevitable.
The "gold standard" is to overbore, epoxy fill, and rebore every hole before reattaching all the tracks, cleats, blocks, rails, and a lot of etc. Use new best quality sealant.
I did this on our boat, and it's tedious but not too technical for a determined amateur. And, I was truly an amateur! In our situation we were also motivated by the need repaint the hull and deck, and restore the old nonskid to safe function again, so that was additional motivation. That, and having nothing better to do with our boat during the worst year of the Pandemic.
Hardest part of the project is probably going to be access to the nuts and washers on the inside. Ask, and I can link your to some photos in my blog.
A further upside is that now we have a "new" sailboat for a fraction of the price of an equivalent new vessel.
Further trivia: I and a buddy just visited the Seattle Boat Show and viewed a new Bene___ 34, and it was less well finished out than your boat and was priced at almost $400K. Cheap-o iron keel, too. Yikes.
Best of luck with your boat! I know several happy owners of that "hull design series" of Catalina (309, 310, 320).
Just opinion, but I believe your model will out-sail a Cat 30, handily.
Three times in the 28 years?? That is amazing.
There may not have been any barrier coat on there. Interprotect 2000 is the standard in the Great Lakes to prevent blisters. It's a two part paint that gets mixed before applying
What happened after year five?
Antifouling paint isn’t really meant to prevent blisters. The higher quality stuff usually just has a higher load of copper or other biocides to prevent barnacles or slime for longer. Blister prevention is usually done with epoxy barrier coats. Without a barrier coat some hulls just do better than others, depending on quality of the original layup and maybe how much time the hull gets to dry between seasons.
In the early years of fiberglass boat building, builders used polyester resin. Polyester resin is cheaper and cures faster than epoxy resin qualities with financial benefits for builders. Polyester resin is, however, not impervious to water which would seep through the resin and react with partially cured resin to form blisters. Once this problem emerged builders switched to Vinylester resin for hulls because it was less expensive and cured faster than epoxy resins and unlike polyester resin it is impervious to water intrusion.
Where were you shopping? West Marine, not noted for low prices, has the most expensive Pettit Antifouling paint for $469 per gallon.
I hauled the boat. Sanded the boat bottom. Painted it with the same paint and splashed her. Ready for the next adventures.
Yeah, at that price point you'd be better off buying a derelict boat with a good hull and deck, and paying someone to rebuild it. You'd end up with a better boat for probably half the price.
I'm sure jealous of you folks up in the North West - here on the Chesapeake, I paint every two years.
Yeah but ... they're engaged in a thread about diesel heaters...
There were a few times a year in Petersburg, FL when members of our cruising club declined sail-outs to anchor destinations (i.e., no plug-in for electric heaters).
; our companions got the one with. The next morning, Christmas Day, the sole of my P30 was so cold that I could not stand long on it w/o me moccasins. Tried to use the Origo (alcohol) stove to raise cabin air temp. Thankfully, our friends invited us over for breakfast where we exchanged gifts in comfortable warm from the portable electric heater they had aboard.. My gift received was a copy of O’Brian’s The Wine Dark Sea (new release). The next day we sailed down the ICW to Clearwater Pass then out into 6-7 ft following seas of the Gulf. By the time we re-entered Tampa Bay is was a beautiful Christmas Day.
Right now I’d take a discussion about diesel heaters over talk about whether the water is still hard.
What paint do you use? I use Pettit Trinidad which is a hard paint. The first couple of years, the slime blocker keeps it completely clean although I still dive the boat every 6-months for zinc and running gear cleaning. The next 6-7 years see a gradual increase in the amount of slime that will aquiter but most of it washes off if I am sailing a lot and it always comes off with just a single swip with my favorite hull cleaning tool which is a blue scotch brite pad on a D handle. It is about4x6 and easy to hold with dive glove. I repaint when I begin to see any type of hard growth which is always mussels that get covered with barnacles but that doesn't happen until year 8-9. I have always had some small areas on the lead of the keel that liked to grow hard stuff but that is kept mostly at bay with twice yearly cleaning. My last haulout, it stripped the keel and refaired it and that fixed most of that issue.
after 
after 
It is not the paint, it is the water that causes a lot of growth. The Chesapeake is much warmer than the waters in the PNW and is very nutrient rich. Ideal conditions for growing all kinds of unwanted plants and animals.