check out a book by scott fratcher called "metal boat repair and maintenance". in it he will tell you that there are various treatments which he describes in detail, and he will tell you how long each treatment will last.
according to him the best way, as in longest lasting, is to blast it and then coat it with amerseal followed by amerlock epoxy
i am in the process of doing this to the inside of my stell hull
. i have an electric compressor
which puts out about 14cfm and i feed it into a pressurised potblaster. i started usinf aluminum
blast medium but am switching to iron silicate as it is a fraction of the price
i have removed all of the loose flaky rust with wirewheels, scrapers, and then sprayed liberally with a strong phosphoric acid (up to 20 times). this was very labor intensive, and i have a feeling that the blaster would have taken care of all that very easily too, so if i was gonna do it again i'd just blast.
remember to have your epoxy sealer (much thinner than the paint
, so it gets deeper into crevices and gives a good bas for the epoxy to adhere to) all ready to go because for best results you really need to cover the fresh streel within a few hours, preferably minutes, or else it starts to rust straight away.
also, the steel needs to be warm, but not hot. if it is a hot sunny day then the metal deck
will get surprisingly hot and the paint
will go off too quicky. i am hoping for a dry, overcast day to paint, where the moisture content of the air is not too high.
according to scott fratcher, if you do it this way then it will last a very very long time.
another tip, if you don't have any limber holes on your horizontals then get a drill and put some in. otherwise and condensation
will collect on the sill and eventually break the paint and then the rust will start all over again.
i am going to glue some plastic spacers to the hull
so that the insulation
will stand off the hull by a 1/4 inch or so, which means the condensation
will fall down into the bilge
(through the limber holes).
the original furring strips were soft wood (unpainted). i'll either paint the new strips with oil
based paint or i may go with plastic instead.
i really recommend the fratcher book. he explains it all very well.
as usual, the more time and effort you put into the prep, the better is the outcome.
i don't really want to rip out my interior
and do this all again in 5-10 years so i'm gonna blast and then use the amerseal/amerlock paint system. they use it on the oil
rigs so it must be pretty good.
good luck. it's a nasty job but done well it will last for years.