I'm going to do my bottom, but im new to this. Also, keep in mind we are talking about a 40 year old boat and money is an important consideration.
There are some scratches and things that look like a bit more than scratches on that need to be filled up. No osmosis, but just damage that goes deeper than the paint and whatever other top layers there may be. Some places, espescially on the bottom of the keel where you are looking at core layers of fibre glass. Other than that, layers of paint and/or gel coat that are bladdering off here and there. Finally, there is one area that has an extensive recent fibreglass repair. (a hole that was fixed) So there it is not the original material that after 40 years i can trust to not blister.
So i will need grind out those holes, fill them up with filler. Thats obvious. If there is no osmosis, how important is it to give it time to dry out and how much time would it need to dry out before i apply anything ? Note that i am in the a tropical climate with humid high temperatures. A am advised to use a mixture of fibreglass resin, fibreglass cut to small pieces and a white powder they name airofibre or something. Sounds good ?
Then comes the layers. I was advised by the shipyard to grind the whole bottom bare to the fibreglass, apply the filler, "paint" the whole thing with polyester resin, then apply barrier coat, then antifouling. Needless to say, this is a hell of a job and the barriercoat adds a bunch to the cost.
Some other boaters out here advised me to just put gelcoat (30$ a gallon right here) on the bare hull and then the antifouling.
Now i am wondering about some things. This is a 40 year old boat that is not blistering, meaning i guess the original fibreglass is good stuff. If i am to do as advised and paint the whole bottom with a cheap fibreglass resin (20$ a galon) do i not actually lower the quality of the top layer and thus possible create a blistering risk that never existed ?
If yes, would it be better, in the areas where there is no damage to sand away only the top layer of antifouling and leave any possible gelcoat layers that may be underneath and apply cheap resin ONLY to the damaged areas where i used filler ?
Then, how big is the difference between barriercoat and gelcoat ? Consider that the barriercoat they offer me is 150$ a gallon and the gellcoat 30$ a gallon. I dont know brandnames, but the price may give an indication of quality... (and for comparison of shop price, they will sell me westmarine 26% copper antifouling for 150$ a gallon
Will gelcoat be enough on a 40 year old hull that has never been blistering ?
It would of course be nice if i could spare a 120$ on using that instead of barriercoat, but more importantly, i read everywhere about the importance of the right processing and layer thickness when applying the barriercoat. I have no means of measuring that exactly, so i worry if i do this unprecise, if it will even be worth using it ? Besides that i wonder about the quality of the barriercoat also because the shipyard does not seem to recognise it as a very important product to prevent blistering, they were telling me the cheap resin layer is what keeps out the water and the barriercoat was initially called a "primer" to make the antifouling stick.
Finally, if the barrier coat is needed, would it be a pro to use gelcoat as well (its ony 30$ anyway....) and if so, which one first ?
Any advice is apreciated, thank you