02-20-2008
|
 |
Senior Member
|
|
Join Date: Sep 2004
Location: Massachusetts
Posts: 999
Rep Power: 8
|
|
|
The general rule of thumb for ferrous metals is to make them think they're not in the water. That means you have to surround them with something that won't let the water get to them. Multi-part paint systems will do the trick for a while, but before you coat the metal you must get it clean and clear of all rust. I'm not familiar with the product you mentioned, but many products with similar advertising claims try to 'convert' the rust before painting it (rather than removing it completely by sand blasting, grinding or similar methods). I'm not a chemist, but I think what these products do is strip off the oxygen molecules from the FeO2, replacing the O2 with something inert -- some of the products use phosporic (sp?) acid, which may make a iron phosphate out of the iron oxide (Caveat: a few lines ago I went well beyond my competence....so take this with a grain of salt).
I've used several of the 'converter' products when I've repaired dings on my boat. I've had mixed results. Some areas I've repaired have never had another problem. Others seem to come back every year or so. I'm not sure what I'm doing wrong, but I think that the cause of the rust relapse is when the converter doesn't get rid of all the oxygen. That or the paint system "leaks" and somehow lets water (with it's complement of oxygen molecules) back in contact with the metal.
All I know is that all the pro's I've seen working on steel boats -- grind or blast the metal white bright before they start painting. The also don't let much time pass between the grind/blast and the first coat of primer. Apparently, oxidation of clean bright metal will begin immediately (I think it's called "flash rust") and the sooner you get the oxygen in the atmosphere out of contact with the metal the better off you'll be.
So, if you want to do it right and not be painting the rusty bits again, get out the grinder, sander, or sand blaster (using appropriate personal and environmental protection gear) and have at it. Have the paint ready and waiting when you've finished grinding.
Also, I've been told it's best to use multi-part paint systems as they chemically bond to the metal and each other. I think it helps if you pick one manufacturer and stick with they're 'system' of primers and topcoats. They're expensive, but I've seen incidents where a manufacturer will refund the product cost when everything is done to their spec and the paint fails. If you mix products (e.g. use POR 15 and then overcoat with an Interlux paint) -- I doubt either POR or Interlux will stand behind the results.
Research the 'paint systems' of Interlux or other well known marine paint companies -- If memory serves, they will have a "paint schedule" for coating iron and steel. Starts with getting the metal bright, then calls for an 'etch primer', then a couple of coats of an epoxy primer and then a top coat, which in the case of a keel will probably be a barrier vinyl coat and then the bottom paint. If you find that you need to fair any divots caused by the rust, the time to do it is between the expoxy primer coats. You apply one coat of primer, then fairing compound --sanded fair, then the second primer coat. (Don't take any of this a gospel -- check it out with the paint supplier, but what's above is probably pretty close to what you'll have to do).
Another place to go for information would be to a commercial boat yard, one that works mostly on metal boats. Ask them what they use and how they go about it.
Done right, you shouldn't have any recurring prolems -- I have a steel hull and I have only a few touch up spots a year below the waterline and a few above -- large sections of the hull have been in the water for over a decade and there's no sign of paint failure or rust.
Last edited by billyruffn; 02-20-2008 at 07:16 PM.
|
Connect with Facebook
