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Technical Articles


 edited oct 2018

by Bob Norson

I know the quote above well because it is mine. It reflects one of the cruel lessons one learns from painting on a steel boat. Another one is; “the first step is the most critical one with each subsequent step decreasing in importance. Both quotes being paradoxically true but the latter doesn’t sound as sexy.

Lets start with a few things I know that don’t work; “Paint right over Rust!” or “Kill Rust”. You find these things in the local hardware store. The clerk will swear by the stuff. Maybe for temporary coverage of a garden mower but don’t waste your time on a boat... Another? “Rust Converting Paint!” or just rust converter. Not up to the job. Iron oxide, rust, is changed chemically when exposed to an acid, becoming iron again. Rust converter is merely a dilute acid and the paints incorporate some of the acid into the mix. Maybe OK for detail work on your old car... I tried one of the best of the breed, “Ironise” by “Gal-Mat” and wound up re-doing all the area I applied it to. I did keep it around for spot repair because it is so easy to apply, being water based, and quickly re-coatable.

In short, a steel boat in a salt water environment is the most severe test of a coating system. If you can blast the steel with abrasives the solutions are far easier. Go right to the best quality epoxy primer and carry on from there. The green police are making it harder to find a place where you can blast and places where you can are quite expensive due to the regulations. So as a practical matter, knowing how to get a reliable paint system repair on weathered steel is a necessity for steel boat ownership.

“It’s the Preparation Stupid!”


Being the proud owner of one of the ugliest box trailers in existence, I found a suitable piece of weathered steel to demonstrate on right in the back yard. (lucky me!) As you can see by the photos, a nice deep scale rusted mess similar to what you find on a neglected steely. The old fashioned hammer and chisel is a good place to start (air powered chisel even better) but be careful of deceit at every step. There is no way a chisel will remove rust suitable for painting. The next step for non-power assisted tools is the screw driver tip scraped vigorously across the area. Better, but not half way there yet. You can succeed in this fashion on small areas but it takes particular attention to detail and way more muscle than you would imagine. If you have magnification available, this is a good time to use it. Every step up in magnification unfailingly reveals a bit or more of scale that missed your attention without it. When you THINK you have it conquered, go over the area firmly with a steel brush and have another look. I bet you find more but should you judge the effort worthy, you are ready to paint, insuring your surface is dry as well as clean. A quick wipe with acetone is a good idea.

If you are in an area that will allow for the noise of an air compressor of reasonable output (10 cfm minimum, 15 better) a scaler tool is more effective and much easier to use.


DO NOT FORGET EAR AND EYE PROTECTION! Go ahead.. ask me how I know but talk really loud, OK!

The makers of the tool generally recommend about 40-60 lbs line pressure to run but I find they hardly work at that pressure. I got away with 80+ but any more and the tool doesn’t last long. The tool makers also recommend a daily oiling of the tool. Be careful of this as any excess oil is blown out the front of the tool. If you know or suspect this has happened, carefully wipe your repair area with acetone or methylated spirits to remove the oil contamination. This may be a good idea in any case as the acetone will tend to remove moisture as well. But I’m getting ahead of myself.


Psychologically it can be hard to persevere because you don’t want to believe it is as bad as it is.. but don’t be fooled.

One of the hardest parts of this is to train yourself to recognise the scale. In photos at bottom, is a spot I missed. I did pick it up when I put the first coat of paint on. I saw the small lump. It was when I put these photos up on the screen that it became more noticeable. With my experience I should have caught it sooner but it appears (sorry) I’m RUSTY!

The lesson learned is you just can’t over-do the preparation. No paint product will save you from serious scaled rust. It will probably come back to haunt you. If I am successful in getting this point alone across to you it’s a win. So.. when you have done it to completion the forth or fith time and finally no new tiny bit of scale explodes in dust from the tool, it’s time to go over it with the steel brush then a thorough clean and dry. Use acetone to wipe down if you think it is possible the surface could still have some oily contamination from the scaler tool... or just because. It never hurts.

Apply your pre-prime paint. POR 15 is the product being used in the photos but it is no longer on the market in Australia and was found to be less than the best with years to compare. Altex pre-prime 167 was the champ but Altex no longer sells it. It appears International Paints has taken it up and kept the number. Ask your International Paint dealer for Pre Prime 167 or Devoe Pre Prime 167. It goes on piss thin and encapsulates surface rust and even light scale rust. The Pre Prime167 has a long dry time, 6 - 8 hours, but likes to be recoated when it is slightly tacky to the touch so consider your timing. Put on at least two coats.




Consider your local climate and season but do try to recoat or overcoat your Pre Prime 167 before it goes hard.

My preference for epoxy primer is Wattyl PR250 because it is cheap and good, a rare combination. Another bonus is that is has a long overcoat tolerance. Up to six months if out of direct sunlight.

You may want to fair the repair. With the first coat of epoxy primer in place it’s a good time to do it. Wattyl Fairing Compound, shown at right, has been discontniued. Use instead, epoxy resin with filler such as phenolic microballons. It goes on smooth and resists air bubbles in the mix and it sands so easy... as long as you don’t let it wait too long. I tried the Jotun stuff as well but I found it harder to work and prone to the bubbles.

For application the best tool I found is a grout spreader for doing tile work. It’s hard rubber blade and wide edge are perfect for the job, just filling the low spots without piling it on everywhere. They are a $5 tool most places.

After the fairing put on your first coat of epoxy undercoat There are many good high build epoxy undercoats. Check local supply and compare costs but I’ve never gone wrong with the Wattyl. At least two coats of undercoat over the fairing. (As applied by brush) As far as top coat, I have over 12 years experience with Wattyl Poly-U-400 and it has been remarkably tough and has the advantage of being easy to re-coat whilst other types of polyurethane have to be sanded or chemically treated to re-coat once cured. At least two coats of top coat as well. That’s 7 coats minimum.


If clear instructions on preparation and use, re-coating times etc, are not on the container then your paint dealer may have the specs on file and will be able to supply you with a copy of the Technical Data Sheet, TDS. Or search the web. The TDS is usually spot on but some details can be fudged. The TDS for Pre Prime 167, branded Devoe, says you must mix the whole kit together but I have had no problem with using small parcels of the paint and storing the sealed cans for reasonable periods. They say you can recoat as long as 72 hours after going hard but the best recoating is done when tacky so that is what I advise. They also give looser qualifications for preparation than specified in this article but... this information is intended for users of steel boats operating in salt water. May your God help you..... because perfection is only 90% effective.


And am I qualified to instruct on this matter?

Well have a look here and judge for yourself..








 The chisel is a good start and quite satisfying to bash around!
 This is the hard way but for a small area and in absence of an air compressor... be relentless!
 The Cool Tool! The scaler bit screws into the main body and the parts are usually purchased separably. I got mine at Super Cheap Auto for a little under $100 all up.
 The scaler is just getting going in this shot but it's useful in showing the hard scale when it is most visible in contrast to the surrounding steel
 At this stage I thought I had got it but not quite. The arrow points to the spot that I will come back to haunt in a year or two but otherwise, this is what it should look like.
 Work the paint in but do not try to leave it very thick. All the brushes I use are cheap throw aways.
 This is where the imperfect spot became visible to me.
In a normal situation and weather window allowing, I would have let this coat set then cleaned out the spot.
 As it says in the main text to left of photos, the fairing compound in the photo is extinct! It was good too, so don't know why. (shakes head) Use any good working filler mix in Epoxy resin, phenolic microballoons are my favourite for this kind of job, or pre-made but insure it is from a reputable maker and make certain it is epoxy. Do not let a salesman talk you into a polyester based car body filler or something. Their favourite lines are; "it's just as good" or "it's the same thing".
 Swipe the grout spreader, loaded with the fairing compound across in a smooth stroke. Avoid sanding by care in spreading. This example was a demo only!! You can make this process as tidy as you like. This example is quick and rough but shows you the steps.
 Several coats of whatever epoxy undercoat you like.... if you are spraying, multiple coatings may not be essential, but never a bad idea.
 Then some good top coat to protect the epoxy and look good! Easy?!?