Has anyone tried Fertan? It uses tannic acid, found in tea. It is completely harmless, free of aggressive substances. Applied to a rusty surface and kept moist for an hour, a black, powdery substance forms within 24 hours. This may be removed with a scouring pad or wet sponge. The resulting surface is inert and the manufacturer claims it may be left for several months  before painting. I have had good results with this stuff.

I struggled to keep the fabricated, mild steel keels of my Kingfisher 26 rust free using epoxy coatings. It is very difficult to find ideal conditions for epoxy unless you are somewhere like Abu Dhabi. I had better results with a moisture-cured urethane primer, which is almost completely insensitive to environmental conditions, provided you don't try and paint over frost.

In my experience maintaining steel hulls, if you use a wire wheel on an angle grinder to take off old paint and rust, and then paint it with epoxy, you will see rust spots bleeding thru the paint within a year.  I believe the problem is that the wire wheel doesn't get bits of rust out of pits, and the rust continues to form under the epoxy. 

If it's not possible to sandblast before putting on the epoxy, then cleaning with a wire wheel on an angle grinder (or just a wire brush if the areas are small), and then painting with POR-15, followed by epoxy, will last many years.  Unfortunately, while it is a great product for steel maintenance, POR-15 (www.por15.com) is not available everywhere. 

Though I have not used it, I have heard good things from a commercial boatyard about Sherwin Williams Corothane, which you should be able to get from any Sherwin Williams dealer (they are in many countries).  Sounding somewhat similar to POR-15, Corothane is also a moisture cured urethane coating, which you topcoat with epoxy.   But their recommended surface preparation is still sandblasting.

I've used several rust converter products, which all seemed to be mostly phosphoric acid (Ospho, Naval Jelly,  Metal Ready).  The rust converters are much easier to find than POR15.  You need to wash the area thoroughly after using these rust converters, then let dry completely, and you can then paint with epoxy. 

I'm not familiar with any rust converters that leave a latex coating behind, so can't really comment on that.   Many epoxy paints thin with xylene, which can dissolve latex paints, so I'd be wary of trying that without asking the manufacturer. 

When I cleaned marine life off my keel, paint came off in places.  Oddly enough, mostly on the wing at the bottom of the keel, while paint on the vertical parts of the keel stayed put.  I have no idea why that is.  Anyway, I started stripping the whole lot down to metal.  There are rust spots after sanding.  I have looked at rust removers.  Some of them are described as leaving behind a latex based primer.  Is that compatible with the epoxy primer I was told to use before putting on antifouling again?