Spot priming steel boat.

majdrew

majdrew

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Hi Folks!

Need some advice..

I need to spot prime some areas below the waterline, trouble is, I don't know what type of primer was used before, and I've been told not to mix differant primers.
The original primer is matt black, maybe coal tar epoxy or something called VC Tar..? Not sure exactly..
Is there something I could use just to spot prime, that wouldn't react with previous primer?

Thanks!
 
If it's flat and black could it have been bitumen paint? You can test that with a bit of petrol or white spirit on a cloth, if it comes off brown......................

My old steel boat was painted in bitumen paint, cheap and easy to re-coat albeit messy.

If your paint doesn't react, try acetone, if still no reaction you're probably safe to put anything on it.

Just my opinion but what I would do.
 
Presumably you're taking it back to bare steel - leave it bare for a day or two and allow it to flash rust then apply neutrarust or one of the other rust neutralisers, you can then prime with primer of your choice once you've decided which one you're going to use. If you fair in the edges of your "spots" and use neutraliser I can't see a problem should one primer come in contact with another -
 
Thanks for the quick replys! The spots are only tiny [blisters] so I would have thought it wouldn't matter too much.. the guy at the chandlery put the jitters up me when I said I didn't know what's on already..! He said to use epoxy filler, but I would be happier with a proper primer..
Thanks again!
 
Please don't use a rust converter - you're building up problems for the future! Adhesion of new anticorrosive will be compromised if you use a rust converter.
You're probably OK with pretty much any primer if your existing primer is aged. You can often get away with an epoxy over a conventional material for small areas of overlap even though it's not an ideal practice.
Wiping the old primer with a strong solvent will give you an idea if it's tar based but even an epoxy tar will stain the solvent rag black.
In your position I would be concerned as to why the old primer is blistering. Is the steel underneath corroded or pitted? If you cut off the top of an unburst blister is the steel shiny?
Have you changed anything on the boat before the blisters started to appear? New electrical equipment, shore power, extra anodes etc?
For a repair I would suggest you wash the affected areas with fresh water to remove any salts, grind back to clean steel and coat with a good zinc rich anticorrosive primer. Build up with a barrier coat before antifouling.
 
Thanks for that.
I'm guessing there is some blistering because when I bought her, she had been moored up a river for a number of years, the water probably being brackish, and the zincs were coated in that white crust, so not protecting her. Or could be too many zincs..
Next haul out I'll have to take a look to see what's happening.. if it's still pitting..
Thanks for the replys!
 
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